Artist now tries harder to find a cover

This is based on !271 (closed) so if you'd prefer the super-small Artist tweak by itself, let me know and I'll cherry-pick it out.

5cf4e846 just does a really simple thing: it fixes the Artist cover art lookup so that it only selects from albums with art. When an artist has multiple albums, it uses a mod operation to pick a cover-of-the-day to use.