OpenSubsonic servers send plural artists/albumArtists refs and can omit the
singular artistId when an album or song has more than one artist. Parse and
store the ordered refs on the item and in indexed album_artists/song_artists
tables, so artist pages, shuffles, downloads and the artist filters include
every credited artist.
Album and now playing artist taps open a chooser when several artists are
credited; context menus get one artist row per artist.
