Implement extra_answer_fields

This code main function is to help 3rd party question types. So probably the best way to test it is to install Preg question type from attached archive preg_25_rc.zip and run question/type/preg/tests/questiontype_test.php which contains advanced tests for user manipulating with both answers and extra_answers_fields. Preg uses options extra_answer_fields, that gives more testing cases than anything else. However, if you don't want to install additional question types, you could test it using shortanswer question. 1. Create shortanswer question with 5 answers. 2. Edit question and see that all 5 answers are in their place on the edit form. 3. Clear 1-st, 3-rd and 5-th answers (you need to clear all answer, fraction and feedback fields to do it) and save question. 4. Edit question and see there are 2 answers on form now (that was 2-nd and 4-th before). 5. Add two new answers in the end and save question. 6. Open question to edit and see that it contains 4 answers for now. 7. Delete 2-nd and add new answer, saving form. 8. Open question to edit and see that there are 4 answers: the ones that were 1-st, 3-rd and 4-th on the 6. step and the new one added there.