3.0 - Create radio widget

Continuing the work for #2267 this set of changes creates a standalone radio button widget. I've still not created any base classes as I want to get a few more implementations under completed before trying to make anything abstract or interfacey.

I've opted to not make hidden fields part of the radio widget as I feel that is a separate concern that FormHelper can orchestrate together. After radio buttons are cleared up, I'll get started on datetime pickers as they are probably the most complex and will require using widgets inside of each other.