MDL-55417 forms: Clean up form element template patch

Use a trait and a base mustache template for all form elements to reduce code
duplication and increase standardisation.

Part of MDL-55071

lib/form/editor.php

@@ -30,7 +30,7 @@ global $CFG;
 require_once('HTML/QuickForm/element.php');
 require_once($CFG->dirroot.'/lib/filelib.php');
 require_once($CFG->dirroot.'/repository/lib.php');
-require_once($CFG->libdir.'/outputcomponents.php');
+require_once('templatable_form_element.php');
 
 /**
  * Editor element
@@ -45,6 +45,10 @@ require_once($CFG->libdir.'/outputcomponents.php');
  * @todo      MDL-29426 ajax format conversion
  */
 class MoodleQuickForm_editor extends HTML_QuickForm_element implements templatable {
+    use templatable_form_element {
+        export_for_template as export_for_template_base;
+    }
+
     /** @var string html for help button, if empty then no help will icon will be dispalyed. */
     public $_helpbutton = '';
 
@@ -451,11 +455,7 @@ class MoodleQuickForm_editor extends HTML_QuickForm_element implements templatab
     }
 
     public function export_for_template(renderer_base $output) {
-        $context = [];
-        $context['frozen'] = $this->_flagFrozen;
-        foreach ($this->getAttributes() as $name => $value) {
-            $context[$name] = $value;
-        }
+        $context = $this->export_for_template_base($output);
         $context['html'] = $this->toHtml();
         return $context;
     }