Allow to uncheck action in QActionGroup

This bug was previously reported many years ago in QTBUG-3512 and closed as not critical, but given the many requests, I decided to open it again.

I have the exact same problem reported by sadesaapuu in QTBUG-3512

"I'm working on a toolbar which has three tools. Only one of them can be enabled at a time, but all of them can be disabled. This is not currently possible with the QActionGroup, and I have to create this simple functionality by hand."

Moreover this feature has been requested multiple times in different forums:

https://stackoverflow.com/questions/22312807/how-to-force-qactiongroup-to-allow-uncheck

https://stackoverflow.com/questions/49180587/how-to-unchecked-all-actions-on-an-action-group-menu-on-qt

https://www.qtcentre.org/threads/32035-QActionGroup-allowing-unchecking

https://forum.qt.io/topic/33054/can-you-think-of-a-better-code-to-uncheck-an-action-of-a-qactiongroup

https://www.qtcentre.org/threads/531-No-action-checked-in-an-exclusive-action-group