* Cleaned up PlaylistInterface. Now basic classes (like the new Album or Artist class) can easily be accessed by the AudioEngine as a proper playlist.
* Added SourceInfoWidget, a summary page for sources.
* Added AlbumView / -Model / -Item.
* WIP: PlaylistManager.
* Many other cleanups / improvements I don't even remember anymore ;-)
Still needs to be added to PlaylistModel---potentially refactored out into somewhere shared.
Also, only allow once column in the source model to be dragged, so we get the right amount of drag items.
