docs: public static CmsSitemapHoverbar installOn(
          CmsSitemapController controller,
          CmsTreeItem treeItem,
          Collection<Widget> buttons) {
  
          CmsSitemapHoverbar hoverbar = new CmsSitemapHoverbar(controller, buttons);
          installHoverbar(hoverbar, treeItem.getListItemWidget());
          return hoverbar;
      }