@Override
public SubmenuToolbarWidget insertSubmenu(int beforeIndex) {
ToolbarToggleButton submenuButton = new ToolbarToggleButton(new VerticalToolbarButtonWidget());
submenuButton.setShowDropdownArrow(true);
panel.insert(submenuButton.hackGetWidget(), beforeIndex);
SubmenuToolbarWidget submenu = new SubmenuToolbarWidget(submenuButton);
// NOTE: don't set action listener or parent on the submenu button, set on
// the submenu itself.
submenu.setParent(this);
items.add(beforeIndex, submenu);