PlaylistTrack: create updateFromFile: method to set attributes

- Just like LibraryTrack's in 6625a3b
- The constructor can now return nil, so Playlist was modified to catch
  that and abort when adding new tracks.
PlaylistTrack: read ALBUMARTIST tag and display this info in the table

- Only set it in the DB if the tag exists and is not blank
- In the Playlist track table:
  - Always use album artist if it exists for the artist
  - Append "// trackartist" if it is different from the album artist and
    the album artist exists
  - This is similar to foobar2000