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 (merged)
Fixes, for example, this "repeat mode" menu in Elisa:
and hundreds of other places.
Edited by ivan tkachenko