Add artist images back, now with proper mosaics.
Slightly tweak the music models, use plural strings when formatting args, and properly sort the music items after everything else is sorted.
Move the size recalculations/child item sorting to a method called finalize() in artists, albums, and genres. Also remove the artist images for now, Ill figure out a good way to bring them back.
Add the Artist item and switch LibraryFragment over to it.
