I created a main window and want to add items and sub-items to the menu bar
but cannot find how to do this. I've looked in the widget box, menus, icons,
Assistant, and web searches without seeing how this is done. The Assistant
has a MenuItem QML Type but I don't know if this is what I want. I also see
in the Assistant's index selections such as menuitem, Menuitem, and menuitems
but those pages do not show me how to add them to the main frame's default
menu or how to add other menu items.