MDL-77997 questions: add back Export as XML to the preview screen

The used the exist in Moodle up to 3.11, but then was removed with
insufficient thought in 4.0 (because we had grander long-term plans
which still have not happened). Until those plans happen, this
commit adds the simple link back on the preview screen.

question/bank/exporttoxml/lib.php

@@ -0,0 +1,46 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Helper functions and callbacks.
+ *
+ * @package    qbank_exporttoxml
+ * @copyright  2023 The Open University
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+use core\plugininfo\qbank;
+use qbank_exporttoxml\helper;
+
+/**
+ * Callback to add content to the question preview screen.
+ *
+ * @param question_definition $question the question being previewed.
+ * @param int $courseid the course id.
+ * @return string HTML to add to the question preview screen.
+ */
+function qbank_exporttoxml_preview_display(question_definition $question, int $courseid): string {
+    if (!qbank::is_plugin_enabled('qbank_exporttoxml')) {
+        return '';
+    }
+
+    if (!question_has_capability_on($question, 'view')) {
+        return '';
+    }
+
+    $exporturl = helper::question_get_export_single_question_url($question);
+    return html_writer::div(html_writer::link($exporturl, get_string('exportonequestion', 'question')));
+}

question/bank/exporttoxml/tests/behat/preview_question.feature

@@ -0,0 +1,27 @@
+@qbank @qbank_previewquestion
+Feature: A teacher can export a question as XML from the preview question screen
+  To help reuse questions
+  As a teacher
+  I can easily export the question I am previewing
+
+  Background:
+    Given the following "users" exist:
+      | username |
+      | teacher  |
+    And the following "courses" exist:
+      | fullname | shortname |
+      | Course 1 | C1        |
+    And the following "course enrolments" exist:
+      | user    | course | role           |
+      | teacher | C1     | editingteacher |
+    And the following "question categories" exist:
+      | name           | contextlevel | reference |
+      | Test questions | Course       | C1        |
+    And the following "questions" exist:
+      | questioncategory | qtype     | name                          |
+      | Test questions   | numerical | Test question to be previewed |
+
+  Scenario: Question preview shows the question and other information
+    When I am on the "Test question to be previewed" "core_question > preview" page logged in as teacher
+    Then the state of "What is pi to two d.p.?" question is shown as "Not yet answered"
+    And "Download this question in Moodle XML format" "link" should exist

question/bank/previewquestion/classes/helper.php

@@ -262,13 +262,12 @@ public static function question_preview_popup_params(): array {
      * @return array
      */
     public static function get_preview_extra_elements(question_definition $question, int $courseid): array {
-        $plugintype = 'qbank';
-        $functionname = 'preview_display';
-        $extrahtml = [];
+        $plugins = get_plugin_list_with_function('qbank', 'preview_display');
+
         $comment = '';
-        $plugins = get_plugin_list_with_function($plugintype, $functionname);
+        $extrahtml = [];
         foreach ($plugins as $componentname => $plugin) {
-            $pluginhtml = component_callback($componentname, $functionname, [$question, $courseid]);
+            $pluginhtml = component_callback($componentname, 'preview_display', [$question, $courseid]);
             if ($componentname === 'qbank_comment') {
                 $comment = $pluginhtml;
                 continue;