This is a patch to do a job. Fairly simple one so I hope review won't be that long.
Things to consider:
1) Due to problem with "answers" field I didn't delete shortanswer code from xml format, so shortanswer question is still importing/exporting directly using format code right now. Hovewer my testing site run without "answers" field for almost a half-year. So I was able to test implementation on shortanswer question type - it is fully interoperable with an existing one in abscence of "answers" field. I did both exporting using new code while importing using old and vice versa. I also diffed xml files generated by existing and new export. All works OK.
Right now extended functionality will be available for any 3d party qtypes using question_extra_fields(). We can drop special handling of shortanswer from format.php after dropping "answers" field.
2) Performance of try_importing_using_qtypes() function was not great from the beginning as it loops throught all qtypes for each question and it will be even worse after implementing import_from_xml function in base questiontype class. I improved this by letting format to make a hint on qtype of the question. You may choose to use this part (changes in two format.php files) or not, it's up to you.
Yes. This would be very useful to have. Thanks.