st-box-layout: Remove insert_actor/insert_before

Now that 'insert_child_at_index' and 'insert_child_below' exist
on ClutterActor, these aren't necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

js/ui/lookingGlass.js

@@ -1038,7 +1038,7 @@ const LookingGlass = new Lang.Class({
             actor.add(padBin);
 
             this._completionActor = actor;
-            this._evalBox.insert_before(this._completionActor, this._entryArea);
+            this._evalBox.insert_child_below(this._completionActor, this._entryArea);
         }
 
         this._completionText.set_text(completions.join(', '));