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!
So the challenge with this group is that the radio buttons for that group are not merely found as under some parent. They also have to share the same `name` attribute. And, if you have a structure like,

```html
<form>
    <input type="radio" name="foo">
    <form>
        <input type="radio" name="foo">
    </form>
</form>
```

...then those two inputs will _not_ be in the same group, even though they would both be found under the same parent element.

And you can't simply just look at direct children because, specifically you don't want them within new `form` contexts. For instance:

```html
<form>
    <input type="radio" name="foo">
    <div>
        <input type="radio" name="foo">
    </div>
    <form>
        <input type="radio" name="foo">
    </form>
</form>
```

Those first type inputs _are_ in the same group, even though one of them is within a div.