Added support for foreign windows to the Win32 backend.

* clutter/win32/clutter-stage-win32.c
	(clutter_stage_win32_request_coords): Don't resize foreign
	windows.
	(clutter_stage_win32_unrealize): Don't destroy foreign windows.
	(clutter_stage_win32_init): Added initialiser for is_foreign_win.
	(clutter_win32_get_stage_from_window): Resort to looking in the
	stage list if the window isn't the right window class so that it
	can still find stages with foreign windows.
	(clutter_win32_set_stage_foreign): New public function to set a
	foreign window for a stage.

	* clutter/win32/clutter-event-win32.c
	(clutter_win32_disable_event_retrieval): New public function to
	disable event retrieval.
	(message_translate): Don't handle WM_SIZE or WM_MOVE for foreign
	windows.

	* clutter/win32/clutter-backend-win32.h (struct
	_ClutterBackendWin32): Added a flag to disable event retrieval

	* clutter/win32/clutter-backend-win32.c
	(clutter_backend_win32_ensure_context): Update debug note to
	include whether the stage is foreign or not.

clutter/win32/clutter-backend-win32.h

@@ -45,9 +45,10 @@ struct _ClutterBackendWin32
 {
   ClutterBackend parent_instance;
 
-  HGLRC gl_context;
+  HGLRC          gl_context;
+  gboolean       no_event_retrieval;
 
-  GSource *event_source;
+  GSource       *event_source;
 };
 
 struct _ClutterBackendWin32Class