Add RadioGroupViewElement impl

First off, I'm not sold on the class name for this one. Second, #getCurrentlySelectedOption I have returning either simply a Radio object or null. Thinking about that while I type this, it probably shouldn't return null but I also don't feel it should return Optional<Radio> -- IIRC, say you have a group of radio buttons: at least one is always selected at some point as opposed to something like a group of checkboxes where all, a few, or none can be checked. Hmm, I dunno. Let me know what you think of that method. Thanks!