1. Cosmetic composition pressurised as an aerosol, characterised in that it comprises in a cosmetically acceptable medium, at least one fixing polymer, at least one non-volatile fluid aryl-silicone, and at least one propellant, the concentration of non-volatile fluid silicone being greater than or equal to 5% by weight relative to the total weight of the composition. 2. Composition according to Claim 1, characterised in that the fixing polymer(s) is (are) present in concentrations of between 0.1% and 20% by weight, and preferably in concentrations of between 1% and 10% by weight, and more particularly between 2 and 8% by weight, relative to the total weight of the composition. 3. Composition according to Claims 1 or 2, characterised in that the non-volatile aryl-silicone(s) is (are) present in concentrations of between 5% and 40% by weight, and preferably between 7 and 30% by weight, relative to the total weight of the composition. 4. Composition according to any one of the preceding Claims 1-3, characterised in that the non-volatile aryl-silicone(s) is (are) present in concentrations of between 10% and 40% by weight, and preferably between 15 and 30% by weight, relative to the total weight of the fluid without propellant. 5. Composition according to any one of Claims 1-3, characterised in that the fixing polymer is chosen from anionic, cationic, amphoteric and non-ionic polymers, and mixtures thereof. 6. Composition according to Claim 5, characterised in that the anionic fixing polymer is chosen from: - polymers containing carboxylic units derived from unsaturated mono- or dicarboxylic acid monomers of formula:   in which n is an integer from 0 to 10, A is a methylene, optionally connected to the carbon atom of the unsaturated group, or to the neighbouring methylene group, when n is greater than 1, via a hetero atom such as oxygen or sulphur, R7 denotes a hydrogen atom or a phenyl or benzyl group, R8 denotes a hydrogen atom or a lower C1-C6-alkyl or carboxyl group, R9 denotes a hydrogen atom, a lower C1-C6-alkyl group or a -CH2-COOH, phenyl or benzyl; - polymers comprising units derived from sulphonic acid, such as vinylsulphonic, styrenesulphonic or acrylamidoalkylsulphonic units. 7. Composition according to the preceding claim, characterised in that the anionic fixing polymer is chosen from: A) acrylic or methacrylic acid homo- or copolymers, or salts thereof, copolymers of acrylic acid and of acrylamide, and salts thereof, sodium salts of polyhydroxycarboxylic acids ; B) copolymers of acrylic or methacrylic acids with a monoethylenic monomer such as ethylene, styrene, vinyl esters, acrylic or methacrylic acid esters, optionally grafted onto a polyalkyleneglycol such as polyethylene glycol and optionally crosslinked; copolymers of this type containing an optionally N-alkylated and/or hydroxyalkylated acrylamide unit in their chain, copolymers of acrylic acid and of C1-C4-alkylmethacrylate; C) copolymers derived from crotonic acid such as those containing vinyl acetate or vinyl propionate units in their chain and optionally other monomers such as allylic esters or methallylic esters, vinyl ether or vinyl ester of a linear or branched saturated carboxylic acid with a long hydrocarbon chain such as those containing at least 5 carbon atoms, it being possible for these polymers optionally to be grafted and crosslinked; D) copolymers derived from C4-C8 monounsaturated carboxylic acids or anhydrides chosen from: - copolymers comprising (i) one or more units of maleic, fumaric or itaconic acids or anhydrides and (ii) units of at least one monomer chosen from vinyl esters, vinyl ethers, vinyl halides, phenylvinyl derivatives, acrylic acid and its esters, the anhydride functions of these copolymers optionally being monoesterified or monoamidated; - copolymers comprising (i) one or more units of maleic, citraconic or itaconic anhydrides and (ii) units of one or more monomers chosen from allylic or methallylic esters optionally containing one or more acrylamide, methacrylamide groups, α-olefin, acrylic or methacrylic esters, acrylic or methacrylic acids or vinylpyrrolidone in their chain, the anhydride functions of these copolymers optionally being monoesterified or monoamidated; E) polyacrylamides containing carboxylate groups. 8. Composition according to Claim 7, characterised in that the anionic fixing polymer is chosen from: - acrylic acid copolymers, such as the acrylic acid, ethyl acrylate and N-tert-butylacrylamide terpolymer; - copolymers derived from crotonic acid, such as vinylacetate terpolymers, tert-butyl-vinylbenzoate and crotonic acid, and crotonic acid, vinylacetate and vinylneododecanoate terpolymers; - polymers derived from maleic, fumaric or itaconic acids or anhydrides with vinyl esters, vinyl ethers, vinyl halides, phenylvinyl derivatives and acrylic acid and esters thereof, such as methylvinylether and monoesterified maleic anhydride copolymers; - copolymers of methacrylic acid and of methylmethacrylate; - the copolymer of methacrylic acid and of ethylacrylate; - the vinylacetate and crotonic acid copolymer; - the vinylacetate, crotonic acid and polyethylene glycol terpolymer. 9. Composition according to Claim 5, characterised in that the amphoteric fixing polymer is chosen from polymers containing units derived from: a) at least one monomer chosen from acrylamides or methacrylamides substituted on the nitrogen atom with an alkyl radical, b) at least one acid comonomer containing one or more reactive carboxylic groups, and c) at least one basic comonomer, such as esters containing primary, secondary, tertiary and quaternary amine substituents, acrylic and methacrylic acids, and the product of quaternization of dimethylaminoethyl methacrylate with dimethyl- or diethylsulphate. 10. Composition according to Claim 9, characterised in that the amphoteric fixing polymer is chosen from the copolymers whose CTFA name is "Octylacrylamide/acrylates/butylaminoethyl methacrylate copolymer" and the methylmethacrylate and methyldimethylcarboxymethylammonioethylmefchacrylate copolymers. 11. Composition according to Claim 5, characterised in that the non-ionic fixing polymer is chosen from: - polyalkyloxazolines; - vinylacetate homopolymers; - copolymers of vinylacetate and of acrylic ester; - copolymers of vinylacetate and of ethylene; - copolymers of vinylacetate and of maleic acid ester; - vinylchloride homopolymers; - polyethylene waxes; - polyethylene/polytetrafluoroethylene waxes; - copolymers of polyethylene and of maleic anhydride; - alkylacrylate homopolymers and alkylmethacrylate homopolymers; - acrylic acid ester copolymers, such as, for example, copolymers of alkylacrylates and of alkylmethacrylates; - copolymers of acrylonitrile and of a non-ionic monomer chosen, for example, from butadiene and alkyl (meth)acrylates; - styrene homopolymers; - copolymers of styrene and of alkyl(meth)acrylate; - copolymers of styrene, of alkylmethacrylate and of alkylacrylate; - copolymers of styrene and of butadiene; - copolymers of styrene, of butadiene and of vinylpyridine; - copolymers of alkylacrylate and of urethane. 12. Composition according to Claim 5, characterised in that the cationic fixing polymer is chosen from: - the copolymer of acrylamide and of dimethylaminoethylmethacrylate quaternized with dimethylsulphate, - copolymers of acrylamide and of methacryloyloxyethyl-trimethylammonium chloride, - the copolymer of acrylamide and of methacryloyloxy-ethyltrimethylammonium methosulphate, - quaternized or non-quaternized vinylpyrrolidone/dialkylaminoalkylacrylate or -methacrylate copolymers, - dimethyl-amino-ethyl-methacrylate/vinylcaprolactam/vinylpyrrolidone terpolymers, and - the vinylpyrrolidone copolymer quaternized by dimethylaminopropylmethacrylamide. 13. Composition according to any one of the Claims 1-12, characterized in that the non-volatile aryl-silicones are chosen from the silicones of the following formula:   in which: R24 which may be identical or different, denotes a C1-C10-alkyl, R26, which may be identical or different, denotes an aryl group, it being possible for this aryl group to comprise one or more optionally substituted aryl rings; R25, which may be identical or different, denotes R26, R24 or Si(R24)3, t ranges from 0 to 1000, u ranges from 1 to 1000, the sum t+u can range from 1 to 2000. 14. Composition according to Claim 13, characterized in that the aryl groups are chosen from phenyl and a phenyl group is substituted with alkyl radicals or C1-C5-alkenyl radicals, and mixtures thereof. 15. Composition according to Claim 14, characterized in that the non-volatile aryl-silicones are chosen from phenyltrimethicone, diphenyldimethicone and phenylmethicone. 16. Composition according to any one of Claims 1-15, characterized in that the cosmetically acceptable medium comprises C1-C6 alcohols. 17. Composition according to any one of Claims 1-16, characterized in that it contains at least one volatile silicone. 18. Composition according to Claim 17, characterized in that the volatile silicone(s) is (are) present in concentrations of between 0,5% and 40% by weight relative to the total weight of the composition. 19. Composition according to any one of Claims 1-18, characterized in that the propellant is chosen from volatile hydrocarbons such as n-butane, propane and isobutane, chlorohydrocarbons and/or fluorohydrocarbons, carbon dioxide, nitrous oxide, dimethylether, nitrogen and compressed air, and mixtures thereof. 20. Composition according to any one of Claims 1-19, characterized in that the propellant is present in concentrations of between 5 and 90% by weight relative to the total weight of the composition. 21. Composition according to Claim 20, characterized in that the propellant is present in concentrations of between 15 and 70% by weight relative to the total weight of the composition. 22. Cosmetic treatment process for keratinous material such as the hair, which consists in app