ScreenShield: lower the shield when the user is idle but before locking

In time span between idle and lock the shield should behave like autologin,
but should prevent accidental reactivation (for example when using a touch
screen) by showing the curtain.

https://bugzilla.gnome.org/show_bug.cgi?id=692560

js/ui/panel.js

@@ -315,7 +315,7 @@ const AppMenuButton = new Lang.Class({
     },
 
     show: function() {
-        if (this._visible || Main.screenShield.locked)
+        if (this._visible)
             return;
 
         this._visible = true;