* Add an instance variable to question_display_options to store the
identifier associated with the question being rendered.
* This information can be used by question type plugins to improve the
accessibility of the answer fields being rendered by adding the
question identifier to the answer fields' labels.
* Adding the question identifier to the label can be achieved by using
question_display_options::add_question_identifier_to_label().

Co-authored-by: Tim Hunt <t.j.hunt@open.ac.uk>