grltrackerplaylists: Add insufficient tagged playlist & remove SongsView

This introduces a playlists that contains songs that are not tagged enough to get displayed in any of the other views.

It also removes songsview.

Issues

• The playlist size is not limited: since we use regular GtkListBoxs for playlists it possibly can not handle the amount of songs.
• No indication of where the offending song/file can be found.