Refactor how radio button groups are handled

[grapoza]

Radio button group initialization (basically the selection of a single radio button from a group) is currently done through a hacky radioGroupValues property on the tree. Internally it's fine, but it's not a great UX for anyone who wants to initialize radio button group selection, and there's no external need for it beyond that since the tree exposes a getCheckedRadioButons method.

A new value should be accepted on the tree view spec for a node to indicate it should be initially selected. This will simplify the prop surface of the TreeView component, and hopefully make tree view radio button initialization more intuitive.