gdm: add a power button

Making users have to log in to power off the machine isn't a good idea.

This commit adds a power menu similar to the one in the fallback greeter
which offers 3 items:

- Suspend
- Restart
- Power off

https://bugzilla.gnome.org/show_bug.cgi?id=657822

js/gdm/consoleKit.js

@@ -0,0 +1,32 @@
+// -*- mode: js2; indent-tabs-mode: nil; js2-basic-offset: 4 -*-
+
+const DBus = imports.dbus;
+
+const ConsoleKitManagerIface = {
+    name: 'org.freedesktop.ConsoleKit.Manager',
+    methods: [{ name: 'CanRestart',
+                inSignature: '',
+                outSignature: 'b' },
+              { name: 'CanStop',
+                inSignature: '',
+                outSignature: 'b' },
+              { name: 'Restart',
+                inSignature: '',
+                outSignature: '' },
+              { name: 'Stop',
+                inSignature: '',
+                outSignature: '' }]
+};
+
+function ConsoleKitManager() {
+    this._init();
+};
+
+ConsoleKitManager.prototype = {
+    _init: function() {
+        DBus.system.proxifyObject(this,
+                                  'org.freedesktop.ConsoleKit',
+                                  '/org/freedesktop/ConsoleKit/Manager');
+    }
+};
+DBus.proxifyPrototype(ConsoleKitManager.prototype, ConsoleKitManagerIface);