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/screen-private.h

@@ -144,6 +144,8 @@ struct _MetaScreen
 struct _MetaScreenClass
 {
   GObjectClass parent_class;
+
+  void (*restacked) (MetaScreen *);
 };
 
 MetaScreen*   meta_screen_new                 (MetaDisplay                *display,
@@ -229,4 +231,6 @@ gboolean meta_screen_apply_startup_properties (MetaScreen *screen,
                                                MetaWindow *window);
 void	 meta_screen_composite_all_windows (MetaScreen *screen);
 
+void     meta_screen_restacked (MetaScreen *screen);
+
 #endif