albumartcache rewrite
The idea is that we just pass a GtkImage + size + media info to the Art class and it updates the image when it is retrieved. This is a bit more involved for treeviews which do not update without explicitly forcing the update on the cellrenderer and actually only take a GdkPixbuf right now.
This cleans a lot of logic from the views.
update
With the intial rework I ran into some problem that were caused by the complicated nature of AlbumArtCache. So split up the logic and implement it more modular and cleanly. I also added a queue for all the types of lookups, like we had before for the iobound lookups.
update (23 january)
More cleanup & rework. Now supports HiDPI images in the searchview (Gtk.TreeView) and does a local lookup for covers in the album directory again (I missed this on the inital rewrite).
update (29 january)
This should be about stable. Added a smooth transition to coverswitching for gtk images.