screenShield: Ensure we destroy background container widgets

We may accidentally leak a widget when monitors change while
locked otherwise. This is especially bad because we put a
box-shadow on this widget.

https://bugzilla.gnome.org/show_bug.cgi?id=697300

js/ui/screenShield.js

@@ -562,6 +562,7 @@ const ScreenShield = new Lang.Class({
             this._bgManagers[i].destroy();
 
         this._bgManagers = [];
+        this._backgroundGroup.destroy_all_children();
 
         for (let i = 0; i < Main.layoutManager.monitors.length; i++)
             this._createBackground(i);