LibraryTrack: use ALBUMARTIST tag

- Add trackArtistName property
- Rename setArtistByName->setAlbumArtistByName to be more clear
- Logic for reading tags:

albumArtist -> (in order of preference) ALBUMARTIST, ARTIST, ""
trackArtist -> nil (if ALBUMARTIST doesnt exist)
               nil (if ALBUMARTIST==ARTIST)
               ARTIST (otherwise)
In othe rwords, trackArtist==nil means regular boring album/track
LibraryView: enable searching trackArtistName too