compositor: The stage is always focused when we're a Wayland compositor

gnome-shell has some complex tracking to set the X input focus
correctly, assuming various things about how the stage is set up in X11.
For instance, it assumes that all actors that get key focus are
gnome-shell Chrome actors that will get events through the stage, so
when one of them is focused, it will try to set the focus back to the
stage.

In Wayland, windows are considered chrome actors that will get key
events through the stage, so this only has the result of unfocusing any
windows that have just received key focus.

We should probably move this input focus moving to mutter instead of
gnome-shell so we can better use mutter's internal state and heuristics.