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.h

@@ -43,23 +43,6 @@
 
 #include "screen-private.h"
 
-/**
- * Layers a window can be in.
- * These MUST be in the order of stacking.
- */
-typedef enum
-{
-  META_LAYER_DESKTOP	       = 0,
-  META_LAYER_BOTTOM	       = 1,
-  META_LAYER_NORMAL	       = 2,
-  META_LAYER_TOP	       = 4, /* Same as DOCK; see EWMH and bug 330717 */
-  META_LAYER_DOCK	       = 4,
-  META_LAYER_FULLSCREEN	       = 5,
-  META_LAYER_FOCUSED_WINDOW    = 6,
-  META_LAYER_OVERRIDE_REDIRECT = 7,
-  META_LAYER_LAST	       = 8
-} MetaStackLayer;
-
 /**
  * A sorted list of windows bearing some level of resemblance to the stack of
  * windows on the X server.