When set to ExclusiveOptional, the new exclusionPolicy property let the
user uncheck the active checkable action in an exclusive group.
[ChangeLog][QtWidgets][QActionGroup] Added new exclusionPolicy
property. Set it to ExclusiveOptional to allow unchecking the active
checkable action in an exclusive group.
Change-Id: I61a9885cfd076d631cddf8c08313e4b488e5dc38
Fixes: QTBUG-71160
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>