windowMenu: Implement new show_menu_for_rect() hook

Having the full geometry of the menu's source button (if any) will
allow us to address several misbehaviors of window menus, so use
that instead of show_menu().

https://bugzilla.gnome.org/show_bug.cgi?id=731058

src/shell-wm-private.h

@@ -45,6 +45,10 @@ void _shell_wm_show_window_menu      (ShellWM             *wm,
                                       MetaWindowMenuType   menu,
                                       int                  x,
                                       int                  y);
+void _shell_wm_show_window_menu_for_rect (ShellWM             *wm,
+                                          MetaWindow          *window,
+                                          MetaWindowMenuType   menu,
+                                          MetaRectangle       *rect);
 
 gboolean _shell_wm_filter_keybinding (ShellWM             *wm,
                                       MetaKeyBinding      *binding);