move Artist.isAlbumArtist to Album.hasAlbumArtist

compare by isSameOnList in Item.merged()
and check for hasAlbumArtist in Album.isSameOnList
fix sorting albums by artists and albumartists
Override listAlbumArtists in CachedMPD
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