lastgenre: Use multi-valued artist field for album matches too

[JOJ0]

Recently multi-valued artist field was put to use in the artist-genre-fetching-stage in lastgenre.

A minor note is that since it only tries to use the albumartists for the fetch_artist_genre, there could be a potential album match with fetch_album_genre for the individual artists (e.g. fetch_album_genre(obj.albumartists[0], obj.album)). Which might have more applicable tags than just falling back to the artists.

But I'm not sure how common that is, and the artist tags are plenty for my use case.

Originally posted by @Nedra1998 in #5981 (comment)

[JOJ0]

@Nedra1998 so we not forget about this feature idea. If you want to give it a try, once #5981 is merged is a good time :-)