Calculated questions: bugs an improvements

Somewhat a permanent issue to centralize bugs and improvements of calculated questions in the user interface, new features, display and problems in quiz, moving through categories, import and export features etc. using the tasks and subtasks feature of the tracker.
If necessary, supplementary docs will be added in
http://docs.moodle.org/en/Development:Calculated_question_bugs_and_new_features_proposal.
and followings