main: Show a warning when gdm is missing

If we are not running under gdm, some functionaliy (such as
the lock screen) does not work, and we should inform the
user about this.

https://bugzilla.gnome.org/show_bug.cgi?id=701212

js/ui/main.js

@@ -268,6 +268,13 @@ function _initializeUI() {
                    _('Running a session as a privileged user should be avoided for security reasons. If possible, you should log in as a normal user.'));
         }
 
+        if (sessionMode.currentMode !== 'gdm' &&
+            sessionMode.currentMode !== 'initial-setup' &&
+            screenShield === null) {
+            notify(_('Screen Lock disabled'),
+                   _('Screen Locking requires the GNOME display manager.'));
+        }
+
         LoginManager.registerSessionWithGDM();
 
         let perfModuleName = GLib.getenv("SHELL_PERF_MODULE");