When adding tracks from Library to Playlist, transfer LibraryTrack objs

- PlaylistView: rename old (add|insert)Tracks functions to
  (add|insert)Tracks...FromFilenames
- PlaylistView: new (add|insert)Tracks...FromLibraryTracks functions
  - These operate on main thread and are blocking as hopefully things
    are fast enough not to need the hack used for filenames
- PlaylistTrack: new function to create a PlaylistTrack from a
  LibraryTrack 
  - Slight subtlety with regards to track/album artist between LT and PT
    so need a conversion there
- This is a lot faster since it doesn't involve reading the tags from
  disk
- Drag and drop still uses old filename method
When drag-drop tracks from Library to Playlist, use LibraryTrack IDs

- PlaylistView needs access to the Library now, to obtain a core data
  object context to get the LibraryTracks using the IDs
- This is a lot faster since it doesn't involve reading the tags from
  disk
PlaylistView: fix typo in header file