Enhancement proposed

Right now, RadioButtons can be created without activating any of the individual buttons, but they can not be cleared once created. It can be achieved in a dirty way, by hacking properties of the associated artists. However, I feel they need to come with a clear() method which deactivates previously selected one - it simply has to do two things: update the button colour and set value_selected to None. If this is acceptable, I'll be happy to send in a PR.