Comment 5 for bug 673237

Update:
* Missing Tracks are not removed from Playlists.
* Tracks are only marked as missing by the library scanner

TODO:
* We should update the missing state at various track operations. Like adding to a playlist, loading to a deck.
* The "Add Random" button should check the path and should not add missing tracks.
* Adding a Missing track to a playlist/crate should be still possible (the user may collect missing tracks for some reasons)
* We should avoid adding missing tracks to Auto DJ. This has party stopper potential.