* Album's id needs to be initialized.
* Artist id must be >0 when initializing it with pre-loaded data.
* Added TrackData and DbCmd_LoadTrackAttributes.
* Added TrackData typedefs.
* Added DbCmd_LoadTrackAttributes.
* Playlist's aboutToBeDeleted is a signal.
* Result shouldn't handle track attributes.
* These DbCmds only want TrackData objects to deal with.
* DRY: No need to manually load track attributes.
* New Track API supports loving without posting to the InfoSystem.
* IdThreadWorker supports loading track IDs.
* InboxJobItem only needs to deal with a TrackData object.
* PlayableItem needs to hook up to the proxied signals.
* The year-property is now part of the track object.
* No need to manually load attributes.
* Track is now proxying a TrackData object (which is cached and only loaded once per track-id).
* TrackData stores a track's data & attributes. It gets cached and proxied by (potentially) multiple Track objects.
* Removed obsolete include.
* DbCmd_ShareTrack operates on a track(-id) basis.
* InboxModel doesn't need to merge / set track attributes anymore.