AnewtFormControlChoice ensuring selection

AnewtFormControlChoice, when not in 'multiple' mode, tries to ensure that a selection is made after setting the value. When an invalid selection was made it silently changes it to the first option. The first bug with this is the case where there are no options to choose from. This is corrected by simply testing if there are any options at all (fixed in anewt.new.cxs).

But I think this behavior is fundamentally flawed. It might be a valid choice when in the code you want to make a default choice for the user with $control->set('value', $container->get('choice')), but that previous choice is no longer valid. In that case you probably want to fall back to the first option or something.

But when the form is seeded by user input this might not be very desirable behavior. There are two cases where I can imagine the value being different from one of the options:
1) A hacker
2) During the time the form is loaded in the browser, someone else changes the options somewhere else, making the user choice invalid.

When dealing with a hacker "any valid choice" is better than "any invalid choice", but in case 2 I'd like the user to get some feedback the option is wrong.

I propose something like a set_default_value($value, $fallback=false) or something to set the value by the code. If $fallback=true, then it can fall back to the first option if no valid option is selected. During user input however, I'd like something to give the user feedback, for example just by selecting nothing, and let get_value() return 'null' or something, so you can handle this situation with a validator. Or do something with is_valid, which sets a default error message in this case (or both).