MDL-38661 Course: Add JS category expander.

This adds a category expanded which:
* fetches child content in a category tree if it has not already been loaded;
* toggles relevant classes on the category node to show and hide child content; and
* applies appropriate animations to improve user experience.

course/category.ajax.php

@@ -0,0 +1,39 @@
+<?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/>.
+
+/**
+ * Helps moodle-course-categoryexpander to serve AJAX requests
+ *
+ * @see core_course_renderer::coursecat_include_js()
+ * @see core_course_renderer::coursecat_ajax()
+ *
+ * @package   core
+ * @copyright 2013 Andrew Nicols
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+define('AJAX_SCRIPT', true);
+
+require_once(dirname(__dir__) . '/config.php');
+
+if ($CFG->forcelogin) {
+    require_login();
+}
+
+$PAGE->set_context(context_system::instance());
+$courserenderer = $PAGE->get_renderer('core', 'course');
+
+echo json_encode($courserenderer->coursecat_ajax());