DRAFT PR

This is an alternative implementation of MenuBar where the menus can be any Widget instead of being required to have MenuItem widgets as menus.

It allow including generic widgets with menu items, but it makes it almost impossible to implement groups (because they can't learn about their context), and it comes with some weird restrictions, like you can't wrap more than one menu item in a collection (Column/Row/Flex, etc.), it can't do tab traversal for the entire tree (because it only sees a subset at a time), and you can't have menu items automatically define the menu shortcuts (because the widgets only exist when they're visible). The alternative is an implementation that only allows a subset of widgets (MenuItem subclasses, where MenuItem implements Widget). It doesn't have any of the above limitations, but you can't just insert a Text into the menu.