MDL-43089 quiz editing: new ajax-y interface.

The goal of this work was to increase usability, and also clean up
the page enough that it will be possible to add new features in future.

Display of mod/quiz/edit.php is now entirely generated by
mod_quiz\output\edit_renderer. This uses a helper class
mod_quiz\structure to provide details of the structure of the quiz, and
mod_quiz\repaginate to alter that structure. (Acutally, there are still
some modification methods on mod_quiz\structure. Expect that to be
cleaned up in future.)

The new code uses much more ajax, and there are new scripts
mod/quiz/edit_rest.php and mod/quiz/repaginate.php to handle this.
(Again, don't be surprised if those two scripts get merged in future.)
Also questionbank.ajax.php (which may, in future, be made more generic,
and moved into the core question bank code.)

Most of the new JavaScript code has intentionally copied the way things
are done when editing activities on the course page.

As a result of this, mod/quiz/editlib.php is now much shorter than it
was. (In future, expect the remaining code in here to move into
mod/quiz/classes.)