Transition to MVC + Utils + Model improvements

Major refactoring:
# Removed QuizCreator
# Controller now adds several listeners for events in the View
# Added helper classes for encapsulation of requests (ICommand, IEvent,
IIndexEvent) and view component (starting with kPrompt. will probably
add more soon).
# Separation of concerns
# Model improvements: IQuiz interface. Now Question types don't need to
have their own answer type - abstracted in to one interface and class
"Answer" serves both types.
Binding to model no longer uses bind keys - now controller takes care of
manipulating model and view.
# Disabled fill-in-blank questions for now during refactoring
# Use of @BXML annotation for referencing non-volatile UI components