Layout child API extensions

- Layout.insertChild(atIndex, child)
- Layout.getChildIndex(child): number

ui/layouts/layout-base.ts

@@ -112,4 +112,4 @@ export class LayoutBase extends view.CustomLayoutView implements definition.Layo
         var layout = <LayoutBase>data.object;
         layout.onClipToBoundsChanged(data.oldValue, data.newValue);
     }
-} 
+}