Make the Radio and Checkboxes field types subclasses of the List type so...

... that getOptions() does not need to be repeated three times.

I realize that there is a minor difference between JFormFieldList::getOptions and the other two classes. But I reckon that it doesn't actually make any difference in practice. 

Also, in the past I would have sent this pull request to the platform but it seems to be closed. I don't really know what's going on, is there a newsletter I could subscribe to or something? Well, if this is inappropriate for CMS, I'll resubmit it elsewhere.