Rework #410
Move volume bar visibility to updateStatus
Typo
avoid NPE in Music.getPath()
remove albumartist user option and make getAlbums return merged lists

- return empty List instead of null in getArtists
Go to Album Artist in NowPlaying
add albumCache and cachedMPD
 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
return songs for any artist if only album name is given

for full albums list when not using cache
adapt equals() for Item merging
rework AlbumCache

- add unique album list to cache (enable listAlbumArtists)
- remove albumsByArtist and albumsByAlbumArtists
- remove artistsByAlbum  and albumArtistsByAlbum