prevent iTunes Control objects from being indexed

They need to be rescanned after a relaunch.
bump version to 2.3.5
allow for a single instance of the QSiTunesDatabase
move trackObjectForInfo:inPlaylist: so it can be used from other sources
correctly implement selection proxies

Doing it this way (with the bundle ID as a key) allows Quicksilver's "Current Selection" proxy object to get actual track and playlist objects. Previously, it would return the file for a track and nothing for a playlist.
bump version to 2.4.0
don't use an iVar since there's only one instance of the library now
don't use an iVar when QSiTunes() will always give the same object
Revert "don't use an iVar when QSiTunes() will always give the same object"

This reverts commit 241660c2fafc2c0abbe3ffa98603a832da45586b.
Merge pull request #16 from quicksilver/indexes

prevent indexing of iTunes controls