code: protected boolean addChild(Widget child, int index, boolean preventLayout) {
          return addChild(child, child.getSceneObject(), index, preventLayout);
      }
docs: Add another {@link Widget} as a child of this one. Overload to intercept all child adds.
  
  @param child
  The {@code Widget} to add as a child.
  @param index
  Position at which to add the child. Pass -1 to add at end.
  @param preventLayout
  The {@code Widget} whether to call layout().
  @return {@code True} if {@code child} was added; {@code false} if
  {@code child} was previously added to this instance.