MDL-55417 forms: Render form elements with a template

This change allows form elements to be overridden with a mustache template.
The template can even listen for form validation errors and supply the JS to
change the look of the form element when there is/isn't a validation error.

Initial support is for all core form elements including:

text, select, selectyesno and checkboxes, groups, dateselector, datetimeselector,
autocomplete, modvisible, advcheckbox, button, duration, filemanager, filepicker, editor, static, grading,
warning, textarea, password, url, submit, questioncategory, recaptcha.

Part of MDL-55071

lib/form/button.php

@@ -26,6 +26,7 @@
  */
 
 require_once("HTML/QuickForm/button.php");
+require_once(__DIR__ . '/../outputcomponents.php');
 
 /**
  * HTML class for a button type element
@@ -37,7 +38,7 @@ require_once("HTML/QuickForm/button.php");
  * @copyright 2007 Jamie Pratt <[email protected]>
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class MoodleQuickForm_button extends HTML_QuickForm_button
+class MoodleQuickForm_button extends HTML_QuickForm_button implements templatable
 {
     /** @var string html for help button, if empty then no help */
     var $_helpbutton='';
@@ -85,4 +86,13 @@ class MoodleQuickForm_button extends HTML_QuickForm_button
             return 'default';
         }
     }
+
+    public function export_for_template(renderer_base $output) {
+        $context = [];
+        $context['frozen'] = $this->_flagFrozen;
+        foreach ($this->getAttributes() as $name => $value) {
+            $context[$name] = $value;
+        }
+        return $context;
+    }
 }