popupMenu: Hide settings menus outside user session

The control-center contains user-pertinent settings
panels. These panels don't make sense to show outside
of a user's session, so hide them for session types other
than SessionType.USER.

https://bugzilla.gnome.org/show_bug.cgi?id=657082

js/ui/popupMenu.js

@@ -813,6 +813,10 @@ PopupMenuBase.prototype = {
     },
 
     addSettingsAction: function(title, desktopFile) {
+        // Don't allow user settings to get edited unless we're in a user session
+        if (global.session_type != Shell.SessionType.USER)
+            return null;
+
         let menuItem = this.addAction(title, function() {
                            let app = Shell.AppSystem.get_default().lookup_setting(desktopFile);