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)