overview: Fix stacking of background and desktop icons

If desktop icons are enabled and not covered by maximized windows,
we will fade them in/out during overview transitions. However when
moving background handling into mutter/gnome-shell, we ended up with
the overview background on top of the DESKTOP window clone, hiding
the fade transition.
Fix the stack order to bring the effect back.

https://bugzilla.gnome.org/show_bug.cgi?id=707671