sessionMode: add a property to disable window manager menus

We will use this to disable menus in the initial-setup session.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/569

js/ui/windowMenu.js

@@ -203,6 +203,9 @@ var WindowMenuManager = class {
     }
 
     showWindowMenuForWindow(window, type, rect) {
+        if (!Main.sessionMode.hasWmMenus)
+            return;
+
         if (type != Meta.WindowMenuType.WM)
             throw new Error('Unsupported window menu type');
         let menu = new WindowMenu(window, this._sourceActor);