Fix IsComposing notification when using CPIM

darcs-hash:20091116104828-56b4d-c189f1e734f4cf8df580a79e8b5c36470cba1124
Made handler on DesktopSharingStream a property

Ignore-this: d17adfdc37ff82c64dfdd8750f25e51b

darcs-hash:20091116114530-b22ec-e0db1b0ac49b91f0a7af6ec976f60a807455cc2c