More cover art stuff

The albumart.org change is self-explanatory.

The other change is so that the noartwork.png image doesn't get loaded for an instant while the real artwork is being transferred from the disk cache into the pixbuf cache.  Now, if the image is in the disk cache but not the pixbuf cache, it will briefly be drawn in background grey before the real image is drawn, or one the image will directly replace the previous one depending on the timing of the draw requests.  I think it is smoother, but see what you think.

Images that aren't in the disk cache will still get the default pixbuf drawn for a short period before the real image appears.  I'm looking at changing this also, but it is more complex and the grey background will be visible for longer so it may not be as pleasing.