Make changes to the Presentation class and WhiteboardNav view (see extended message)

Presentation:
   Add True/False return value depending on whether preconditions were met for methods
    MoveToNextSlide(), MoveToPreviousSlide(), and MoveToSlide().
   Remove unnecessary SetPath() checks.
   Update documentation.

WhiteboardNav:
   Make it so the whiteboard and current slide are only refreshed when the slide was
    successfully changed.
   Make it so the slide number text box only allows numbers to be entered.