Question List Mode implementation

This uses tabulated-list-mode to build the list of questions.
The actual content of questions is displayed on a separate buffer using org-mode (for now of course, this is not permanent).
By when reading your part of the code, I didn't spot anything which would be hugely improved by cl-lib. There are a couple of spots where we *can* use it, but nothing where we *should*.

The biggest one would have been that filter function we discussed so much about, which could have been implemented identically with `(cl-member-if (lambda (x) (or (eq x item) (eq (car-safe x) item))) tree)`. But, of course, we've already changed that with an `or+member+assoc` (which I think will be faster in general) so there's no need for it now.