docs: public void setHeader(String header) {
          headerLabel.getElement().setInnerSafeHtml(SafeHtmlUtils.fromString(header));
          addStyleName(CssName.WITH_HEADER);
          ListItem item = new ListItem(headerLabel);
          UiHelper.addMousePressedHandlers(item);
          item.setStyleName(CssName.COLLECTION_HEADER);
          insert(item, 0);
      }