Add a MetaScreen:restacked signal and expose MetaWindow.layer

Expose restacking and a window's stack layer to allow a compositor
to insert elements into the window stack in the right location.
(See Bug 571827 – hide panel when screensaver is active)

src/core/stack.h src/include/common.h: Move MetaStackLayer to
 a public header.

src/core/screen.c src/core/screen-private.h src/core/stack.c:
 Add a ::restacked signal emitted after we finish restracking.

src/core/window.h src/include/window.h: Add meta_window_get_layer()

src/core/window.c

@@ -8673,3 +8673,8 @@ meta_window_get_role (MetaWindow *window)
   return window->role;
 }
 
+MetaStackLayer
+meta_window_get_layer (MetaWindow *window)
+{
+  return window->layer;
+}