MDL-65029 questions: refactor generation of question div id

We are planning to change the id in Moodle 3.7. However, the new
method name has been back-ported to stable branches, because we
know a lot of third-party question types like to have a single
branch which supports multiple Moodle versions.

question/engine/questionattempt.php

@@ -359,6 +359,19 @@ class question_attempt {
         return 'q' . $this->usageid . ':' . $this->slot . '_';
     }
 
+    /**
+     * When the question is rendered, this unique id is added to the
+     * outer div of the question. It can be used to uniquely reference
+     * the question from JavaScript.
+     *
+     * Note, this is not truly unique. It will be changed in Moodle 3.7. See MDL-65029.
+     *
+     * @return string id added to the outer <div class="que ..."> when the question is rendered.
+     */
+    public function get_outer_question_div_unique_id() {
+        return 'q' . $this->slot;
+    }
+
     /**
      * Get one of the steps in this attempt.
      *