Keep the "default" symlink for XFCE in the kali-themes-common package.
We need to embed the version in the package name if we want the old
wallpapers to coexist with the new ones on user's systems to avoid
breaking their non-default backgrounds.
Hopefully this one will never change. It means we have the same image
twice but I prefer this over not having any background image at all when
we drop the old background.
