MDL-75910 core_form: Allow editor field to set a hidden label

When an editor is used in a form as part of a group, it is missing its
label element.

This happens because there is no inline template for the editor, and
therefore it reverts to using the old `toHtml` function. The legacy
function does not cater to the editor being in a group, and therefore
does not add the hidden label.

In addition, the hiddenLabel attribute was missing from the editor
element. This is a semi-standard element but must be added to each
supporting element.

Both the missing inline template for the editor, and the hidden label
attribute must be present:
- If the hidden label attribute is not present, then the standard inline
  template will add a visible label within the group.
- If the inline editor template is missing, then the label is not shown
  at all.

lib/form/editor.php

@@ -64,6 +64,9 @@ class MoodleQuickForm_editor extends HTML_QuickForm_element implements templatab
     /** @var array values for editor */
     protected $_values     = array('text'=>null, 'format'=>null, 'itemid'=>null);
 
+    /** @var bool if true label will be hidden */
+    protected $_hiddenLabel = false;
+
     /**
      * Constructor
      *
@@ -496,4 +499,14 @@ function getFrozenHtml() {
 
         return '';
     }
+
+    /**
+     * Sets label to be hidden.
+     *
+     * @param bool $hiddenLabel Whether the label should be hidden or not.
+     * @return void
+     */
+    function setHiddenLabel($hiddenLabel) {
+        $this->_hiddenLabel = $hiddenLabel;
+    }
 }

lib/form/templates/element-editor-inline.mustache

@@ -0,0 +1,5 @@
+{{< core_form/element-template-inline }}
+    {{$element}}
+        {{{element.html}}}
+    {{/element}}
+{{/ core_form/element-template-inline }}