RadioButton: fix box-sizing in the theme

RadioButton: fix box-sizing in theme such that it works nicely even when using delite/themes/defaultapp.css which sets margin-box on all elements. 

See #408.
> 408?

Yes, but for the moment my PR is on hold, since it was said a different solution would be preferrable. If we'll finally decide to go with the solution in this PR, I'll fix the issue number in the comment.