gdm: disconnect signals

Many signal connections on global objects and on non-widgets were not
disconnected when the unlock screen was destroyed, causing leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=738256

js/gdm/authPrompt.js

@@ -134,8 +134,7 @@ const AuthPrompt = new Lang.Class({
     },
 
     _onDestroy: function() {
-        this._userVerifier.clear();
-        this._userVerifier.disconnectAll();
+        this._userVerifier.destroy();
         this._userVerifier = null;
     },