CheckIndicator: Allow exclusive buttons to be detected via their ButtonGroup

This is a bit wasteful, as it creates a ButtonGroup attached object for
every menu item that does not have it. But it works. Unfortunately,
there is no better approach, as if ButtonGroup is bound to an item
later than this binding is established, we won't know about it.

All components using CheckIndicator, except MenuItem, were modified to
override this new more advanced heuristic with a fixed elementType, so
they don't waste memory on attached objects, and always have
predictable appearance.

See also: kirigami!1014

BUG: 467390