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).
This dictates an enumeration but can they always submit multiple values back to the server?