Removing Shorcuts.of and Shortctus.maybeOf (#104215)

This removes Shorcuts.of and Shortctus.maybeOf because they're not especially useful, since the only thing you can really set on a ShortcutManager is the shortcuts, and the Shortcuts widget that you give it to manages those, so if it rebuilds, it overwrites what you set.

Also, adds a Shortcuts.manager constructor and removes the manager argument to the Shortcuts widget.

Removing these will also eliminate an InheritedWidget for each Shortcuts widget, improving memory usage.