Support (fallback) app menu in SSD

We now allow "appmenu" in the button layout to make synchronizing it
with GTK+'s client-side decorations easier, but as some people tweak
their settings to get in-window app menus even when using the shell,
actually pop up the app menu when the button is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=730752

src/shell-wm-private.h

@@ -42,6 +42,7 @@ void _shell_wm_show_tile_preview     (ShellWM             *wm,
 void _shell_wm_hide_tile_preview     (ShellWM             *wm);
 void _shell_wm_show_window_menu      (ShellWM             *wm,
                                       MetaWindow          *window,
+                                      MetaWindowMenuType   menu,
                                       int                  x,
                                       int                  y);