Now that we have RTTI in userspace, we can do away with all this manual
hackery and use dynamic_cast.
We keep the is<T> and downcast<T> helpers since they still provide good
readability improvements. Note that unlike dynamic_cast<T>, downcast<T>
does not fail in a recoverable way, but will assert if the object being
casted is not a T.
Making an AbstractButton exclusive means that we enforce that only one
of the exclusive buttons within the same parent widget can be checked
at a time.
RadioButton was doing exactly the same thing, except in a custom way.
So just remove the custom code and make it exclusive. :^)
