Allow the shell to run without the screenshield

The screenshield requires gdm 3.5, which can be problematic in
jhbuild configurations, or distributions that don't use GDM as the display
manager. Allow transparent fallback to gnome-screensaver in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=683060

js/ui/unlockDialog.js

@@ -2,7 +2,7 @@
 
 const AccountsService = imports.gi.AccountsService;
 const Clutter = imports.gi.Clutter;
-const Gdm = imports.gi.Gdm;
+const Gdm  = imports.gi.Gdm;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Gtk = imports.gi.Gtk;
@@ -23,6 +23,35 @@ const GdmUtil = imports.gdm.util;
 // The timeout before going back automatically to the lock screen (in seconds)
 const IDLE_TIMEOUT = 2 * 60;
 
+function versionCompare(required, reference) {
+    required = required.split('.');
+    reference = reference.split('.');
+
+    for (let i = 0; i < required.length; i++) {
+        if (required[i] != reference[i])
+            return required[i] < reference[i];
+    }
+
+    return true;
+}
+
+function isSupported() {
+    try {
+        let params = GLib.Variant.new('(ss)', ['org.gnome.DisplayManager.Manager', 'Version']);
+        let result = Gio.DBus.system.call_sync('org.gnome.DisplayManager',
+                                               '/org/gnome/DisplayManager/Manager',
+                                               'org.freedesktop.DBus.Properties',
+                                               'Get', params, null,
+                                               Gio.DBusCallFlags.NONE,
+                                               -1, null);
+
+        let version = result.deep_unpack()[0].deep_unpack();
+        return versionCompare('3.5.91', version);
+    } catch(e) {
+        return false;
+    }
+}
+
 // A widget showing the user avatar and name
 const UserWidget = new Lang.Class({
     Name: 'UserWidget',