code: protected void add(Widget child, Element container) {
  
          // Detach new child.
          child.removeFromParent();
  
          // Logical attach.
          getChildren().add(child);
  
          // Physical attach.
          DOM.appendChild(container, child.getElement());
  
          // Adopt.
          adopt(child);
      }
docs: Adds a new child widget to the panel, attaching its Element to the
  specified container Element.
  
  @param child the child widget to be added
  @param container the element within which the child will be contained