win32: destroy clipboard notification window on dispose

The clipboard uses a hidden window to get some specific events.
The window was created but never destroyed on dispose.

gdk/win32/gdkdisplay-win32.h

@@ -32,6 +32,7 @@ struct _GdkWin32Display
   GHashTable *cursor_cache;
 
   HWND hwnd;
+  HWND clipboard_hwnd;
 
   /* WGL/OpenGL Items */
   guint have_wgl : 1;