WIP commit to illustrate one approach to issue #573. No DB changes....

doc/en/Authoring/Potential_response_trees.md

lang/en/qtype_stack.php

question.php

questiontype.php

renderer.php

settings.php

stack/potentialresponsetree.class.php

tests/helper.php

tests/potentialresponsetree_test.php

tests/walkthrough_adaptive_test.php

tests/walkthrough_deferred_cbm_test.php

tests/walkthrough_deferred_feedback_test.php