In quiz mode: Explanation of previous question

Issue #32

When in quiz mode, if a question is answered correctly, offer the user
to view the explanation. Don't expand it by default, in order to not
disturb the user.

Had to remove the fading effect from the "Correct!" h1, as it was now
just confusing.

Does not work with the last question yet.
Dead code
Allow user to view explanation for previous question, also for the last question.

Issue #32