Add the window overlays we're currently showing using the WindowOverlay
class to the WindowClone class and implement them using
ClutterConstraints instead of the old fixed position/size layout, which
had to be used because the workspaces were scaled, and the title and app
icon were kept unscaled using a separate layer.
Specifically, this is done by adding the ClutterClones to a static
container owned by the WindowClone and adding the elements of the
overlay as children to the WindowClone itself. That way the
overlay-elements can use the container as a source for their constraints
and we avoid having to make sure the overlays remain visible above the
ClutterClones.
We're not using the new overlays yet, they're hidden by default and
showOverlay() isn't called anywhere yet.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1298
33ab53068e