widgets with dynamic-choice

[Ted Zlatanov]

I've needed widgets with dynamic choices many times for Customize.
Stefan also suggested a `dynamic-choice' would be handy. So, I wanted to
ask some questions before we write code:

* what should it look like? Just a callable symbol or a form or
  something else?

* when should the widget get updated?

* how do we deal with outliers like a dynamic 100000 element list?

* how are errors handled? do we empty the list or go back to the last
  good version?

* should this just be a way to provide 'choice and 'radio, or do we want
  a more generic facility that can handle the other composite types?

Ted