When I was doing the MenuBar implementation, I made some changes to the PlatformMenuBar to allow it to understand shortcuts a little more, and to deprecate the body parameter rename it to child to match most other widgets.
These are those changes, separated out because they are separable, and I'm trying to make the MenuBar PR smaller.
Implements a PlatformMenuBar widget and associated data structures for defining menu bars that use native APIs for rendering.
This PR includes:
A PlatformMenuBar class, which is a widget that menu bar data can be attached to for sending to the platform.
A PlatformMenuDelegate base, which is the type taken by a new WidgetsBinding.platformMenuDelegate.
An implementation of the above in DefaultPlatformMenuDelegate that talks to the built-in "flutter/menu" channel to talk to the built-in platform implementation. The delegate is so that a plugin could override with its own delegate and provide other platforms with native menu support using the same widgets to define the menus.
This is the framework part of the implementation. The engine part will be in flutter/engine#32080 (and flutter/engine#32358)
