add signal checkedButtonChanged() to QButtonGroup

For exclusive QButtonGroup I would like to react to the change of checked button, with information which button got checked (and maybe which got unchecked).

Right now we get only signals about pressed/released/clicked, which will not be emitted when a button in the group gets checked from the code (right?).

The best probably would be to have both:

void checkedButtonChanged(int new_id, int old_id);

and

void checkedButtonChanged(QAbstractButton * new, QAbstractButton * old);