SubmenuToolbarView submenu = toolbar.addSubmenu();
new ToolbarButtonViewBuilder()
.setIcon(css.heading())
.applyTo(submenu, null);
submenu.setShowDropdownArrow(false); // Icon already has dropdown arrow.
ToolbarClickButton defaultButton = submenu.addClickButton();
new ToolbarButtonViewBuilder()
.setText("Default")
.applyTo(defaultButton, createClearHeadingsListener());
ToolbarView group = submenu.addGroup();
for (int level : asArray(1, 2, 3, 4)) {