Factored MultipleChoiceInput into MCInput, the base of multiple choice format input modes and MultipleChoiceMCInput, the actual multiple choice mode itself.

This allowed me to begin the article test, using MCInput as a base.

So far, the GUI works, but the AnswerValidator needs to be changed to correct correctly (heh), and fregl and I need to discuss some more about a KEduVocWordType/KEduVocArticle api change.

So, in summary: Multiple choice works like before and article test works, but you'll get everything wrong. ;)