stack: Stack both wayland and X hidden windows below the guard window

Stacking hidden X windows below the guard window is a necessity to
ensure input events aren't delivered to them. Wayland windows don't
need this because the decision to send them input events is done by us
looking at the clutter scene graph.

But, since we don't stack hidden wayland windows along with their X
siblings we lose their relative stack positions while hidden. As
there's no ill side effect to re-stacking hidden wayland windows below
the X guard window we can fix this by just doing it regardless of
window type.

https://bugzilla.gnome.org/show_bug.cgi?id=764844