A MenuToolButton is a ToolItem that contains a button and a small additional button with an arrow.
When clicked, the arrow button pops up a dropdown menu.
Use MenuToolButton to create a new MenuToolButton.
The GtkMenuToolButton implementation of the GtkBuildable interface supports adding a menu by specifying “menu” as the “type” attribute of a `<child>` element.
An example for a UI definition fragment with menus:
<object class="GtkMenuToolButton">
<child type="menu">
<object class="GtkMenu"/>
</child>
</object>