Multiple values are now a separate question type...

... this simplifies everything, including the types of UI Hints there are for each type of question, and constraints are either specific to a type, or an enumeration if there are multiple values. This is simpler and doesn't model as much, but it does cover everything I've seen so far. Will probably need to add a flag to the multiple value type as to whether the user can enter another value or not ("Other" category).
Remove this: it's not used.