Experiment changes to the Collection Browser:
* Display (up to) 3 covers for artists
* To fit them in, show them to the right of the text
* For consistency, move the cover for an album there too
* Blank covers are not shown for artists, but are shown for albums
* Covers should be (implicity) shared by Qt.

The aesthetics could probably be improved, but I don't quite know how.
Personally, I'm not even sure I like it at the moment.

Credit to the idea goes to Theory