MDL-41885 modules now declare use of question bank

* FEATURE_USES_QUESTION_BANK is now a module_supports flag which declares
  that an activity uses the question engine.

* question_module_uses_questions can be used to determine if a module
  uses the question bank.

lib/moodlelib.php

@@ -433,6 +433,9 @@ define('FEATURE_BACKUP_MOODLE2', 'backup_moodle2');
 /** True if module can show description on course main page */
 define('FEATURE_SHOW_DESCRIPTION', 'showdescription');
 
+/** True if module uses the question bank */
+define('FEATURE_USES_QUESTIONS', 'usesquestions');
+
 /** Unspecified module archetype */
 define('MOD_ARCHETYPE_OTHER', 0);
 /** Resource-like type module */