user-status: Hide "Power off" option if shutdown is disabled

Due to lockdown settings or Polkit policy, shutdown may not be
available. If this is the case, the "Power off ..." action should
be hidden from the user status menu.

https://bugzilla.gnome.org/show_bug.cgi?id=652038

js/misc/gnomeSession.js

@@ -109,7 +109,8 @@ const SessionManagerIface = {
     name: 'org.gnome.SessionManager',
     methods: [
         { name: 'Logout', inSignature: 'u', outSignature: '' },
-        { name: 'Shutdown', inSignature: '', outSignature: '' }
+        { name: 'Shutdown', inSignature: '', outSignature: '' },
+        { name: 'CanShutdown', inSignature: '', outSignature: 'b' }
     ]
 };