storage : add an init function

needed to initialise the lua parts
film : make some existing functions visible to the rest of the world
j2k : create a real enum for the presets

this is needed for lua that needs the enum names
gallery : make a function to calculate storage size

the storage size reported to the outside world is not the real size
this is recalculated multiple time, so just create a function for that
Merge pull request #213 from darktable-org/lua_review

storage : add an init function