Implement window menus in gnome-shell

https://bugzilla.gnome.org/show_bug.cgi?id=726352

src/shell-wm-private.h

@@ -40,6 +40,8 @@ void _shell_wm_show_tile_preview     (ShellWM             *wm,
                                       MetaRectangle       *tile_rect,
                                       int                  tile_monitor);
 void _shell_wm_hide_tile_preview     (ShellWM             *wm);
+void _shell_wm_show_window_menu      (ShellWM             *wm,
+                                      MetaWindow          *window);
 
 gboolean _shell_wm_filter_keybinding (ShellWM             *wm,
                                       MetaKeyBinding      *binding);