Questions can have negative position numbers

[HirotoKagotani]

I had the same situation as a discussion topic https://moodle.org/mod/forum/discuss.php?d=390636 and I figured out how to give negative position numbers to questions.

I am using Questionnaire version 4.1.1 on Moodle 4.1.

Reproduction steps:

1. Add a Questionnaire Q1 with one or more question(s).
2. Add another Questionnaire Q2 with:
   • a question.
   • a page break.
   • any number of question(s).
3. Move the 1st question of Q2 to the next position of the page break.
4. The page break is deleted by the code in class/questions_form.php.
5. You see the position number of the 1st question of Q1 is 0. (Also the position numbers of ALL questions of ALL questionnaires are decreased by one)
6. Add a page break to Q2.
7. Move the page break to the 2nd position.
8. Move the 1st question of Q2 to the next position of the page break again.
9. You see the position number of the 1st question of Q1 is -1.
10. and so on...

Also we need a method to safely reset the position numbers for all questionnaires (in a Moodle) from 1.