Improve shuffling of song with "featured" artists

[derat]

The shuffling changes in #28 were successful in making it unlikely that two songs by the artist "Foo" will be played close together while shuffling, but a song by "Foo feat. Bar" will be unaffected.

At first I thought that maybe there's some way that I can form clusters by also including album names. That won't help in this case where there's a song by artist 'Foo' from album 1 and a song by artist 'Foo feat. Bar' from album 2 (unless we also happen to be shuffling a song by artist 'Foo' from album 2).

Maybe I should instead (or additionally) use AlbumArtist when it's set, or do some sort of prefix matching.