PlaylistTrack: artistName->trackArtistName, +albumArtistName (model v2)

- Playlist v2 Core Data model
- Rename artistName -> trackArtistName (to be crystal clear)
  - To do this involves deleting the old property and adding a new one
    that has the renamingIdentifier set to the previous property name
- Add albumArtistName
- Update PlaylistTrack accessors for these two new fields
  - Update the tag reading code to set trackArtistName
  - Update the cell view to use trackArtistName

NB: reading/setting albumArtistName not done yet.