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/stack.c

@@ -1287,6 +1287,8 @@ stack_sync_to_server (MetaStack *stack)
     g_array_free (stack->last_root_children_stacked, TRUE);
   stack->last_root_children_stacked = root_children_stacked;
   
+  meta_screen_restacked (stack->screen);
+
   /* That was scary... */
 }