Re-works the approach to supporting live preview to handle stacking.

We can't easily use a metacity layer to hide windows as that means we
loose our original stacking position. (Metacity's stack positions are
only valid within a single layer) We now have a "guard window" per
screen that is a fullscreen override redirect that is lowered to the
bottom of the stack. Hidden windows now remain in their original layer
so the stacking position remains valid, but all hidden windows get
XRestacked under the guard window.

A new compositor hook is also added to inform it when a window becomes
hidden/unhidded, this lets us map/unmap the corresponding actor.

missing files in preview commit (TODO: rebase -i and squash this later)

src/core/window-private.h

@@ -428,9 +428,6 @@ void        meta_window_resize_with_gravity (MetaWindow  *window,
                                              int          gravity);
 
 
-/* Return whether the window would be showing if we were on its workspace */
-gboolean    meta_window_showing_on_its_workspace (MetaWindow *window);
-
 /* Return whether the window should be currently mapped */
 gboolean    meta_window_should_be_showing   (MetaWindow  *window);