Initially we went with a RadioSet being a simple container of RadioButtons,
with the user navigating the RadioButtons like you would any other set of
widgets. This was fine but it became pretty clear pretty quickly that having
to tab through a non-trivial collection of buttons in a set to get to the
next widget wasn't ideal.
This commit, satisfying #2368, takes over the navigation of the buttons
within the container, makes the container itself a focusable widget, and
sets up some new bindings to allow a more natural and efficient interaction
with the set.
Unlike a few other widgets, the RadioSet is pretty much all about reacting
to the selection result; the question of how you go about it has already
come up and while the message is documented, complete with all properties,
it can't hurt to have an illustrative example of code that uses it.
Here I add an extra RadioSet example that sits with the message in the
reference. This should help the reader better follow how to use it, and also
gives something to link to if someone hasn't got that far into the
documentation yet but is attempting to use the RadioSet.
