The logic in quiz_questions_in_use is wrong.

Quizzes can use questions in two ways:

1. Because that question appears in a quiz attempt. That bit of the logic, computed via question_engine::questions_in_use, is right.
2. Because a quiz_slot refers to the question, via question_references. This bit is wrong.

The reference is relevant if:

• The reference is to a specific question_version, and the questionid is that exact version.
• The reference is to 'always latest' and the questionid is the latest non-draft version of that question.

In fact, this logic should not be in the quiz callback at all. Instead, question_references should be handled in core questionlib.php.

While working on this, I found some other issues in the code:

1. Some of the unit tests were asserting the wrong delete behaviour for unused versions. I fixed that.
2. The logic about hiding versions, rather than deleting them, if they are in use, was split between the delete script and the back-end function. I consolidated all that logic in the question_delete_question API function.
3. I fixed some surprising behaviour in core_question_generator, which confused me while working on this.
4. I fixed the poor PHPdoc comment on question_require_capability_on.