This makes checkable buttons exclusive with other checkable buttons in the
same parent widget. Basically like radio buttons. This should probably be
used to implement GRadioButton once it's a bit more mature.
This is done by dispatching a (synchronous) "EnabledChange" event that can
be picked up in change_event(). Use this event to kick widgets out of their
"being pressed"-type modes if the user is interacting with them while the
state is programmatically changed.
