Add extra_question_fields() support to backup/restore functions

backup/restore function of default_questiontype class never support extra_question_fields() , thought a commentary before extra_question_fields() boldly states it does.

Adding such functionality allows us to make backup/restore in shortanswer use extra_question_fields(), and so become more inheritable.

The issue seems to be pretty easy, so I'm probably be able to do it by myself (I'm not talking of extra_answer_fields() support, just extra_question_fields() ). Hope you'll appreciate it and converting shortanswer to use it.