move Artist.isAlbumArtist to Album.hasAlbumArtist

- adapt equals() for Item merging
- fix sorting albums by artists and albumartists
rework AlbumCache

- add unique album list to cache (enable listAlbumArtists)
- remove albumsByArtist and albumsByAlbumArtists
- remove artistsByAlbum  and albumArtistsByAlbum
replace MPD by CachedMPD (without enabling it)
add option whether to use local album cache
allow AlbumsGridFragment for full album list when using cache
remove Artist from SongsFragment
adjust Searchactivity
show artists in albums list
return songs for any artist if only album name is given

for full albums list when not using cache