Desktop Background Capplet Previews

I’ve created a patch which fixes jdub’s request #1 (and bug 139714) using the GNOME thumbnailing facilities. I’m not sure if GnomeThumbnailFactory currently outputs any metadata on thumbnails (which is required to show things like width and height), but it definately should if it doesn’t.

Update: GnomeThumbnailFactory saves some metadata, just not the original images’ width/height. Patch to fix this at bug 143470.