code: protected CmsToggleButton createOpener() {
  
          final CmsToggleButton opener = new CmsToggleButton();
          opener.setButtonStyle(ButtonStyle.FONT_ICON, null);
          opener.setSize(Size.small);
          opener.addStyleName(CSS.listTreeItemOpener());
          opener.setUpFace("", I_CmsButton.TREE_PLUS);
          opener.setDownFace("", I_CmsButton.TREE_MINUS);
          opener.addClickHandler(new ClickHandler() {
  
              /**
               * @see com.google.gwt.event.dom.client.ClickHandler#onClick(com.google.gwt.event.dom.client.ClickEvent)
               */
              public void onClick(ClickEvent e) {
  
                  setOpen(opener.isDown());
                  e.stopPropagation();
                  e.preventDefault();
              }
          });
          return opener;
      }
docs: Creates the button for opening/closing this item.<p>
  
  @return a button