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.
