MDL-80192 forms: implement method to set group element hidden label.

This is consistent with the API used for other form element types,
in order to provide screen-readable text in the fieldset legend while
not necessarily wanting to display said label.

The new class property is automatically exported to the template via
the `templatable_form_element` trait.

lib/form/group.php

@@ -46,6 +46,9 @@ class MoodleQuickForm_group extends HTML_QuickForm_group implements templatable
     /** @var string html for help button, if empty then no help */
     var $_helpbutton='';
 
+    /** @var bool if true label will be hidden. */
+    protected $_hiddenLabel = false;
+
     /** @var MoodleQuickForm */
     protected $_mform = null;
 
@@ -104,6 +107,15 @@ class MoodleQuickForm_group extends HTML_QuickForm_group implements templatable
         }
     }
 
+    /**
+     * Sets label to be hidden
+     *
+     * @param bool $hiddenLabel sets if label should be hidden
+     */
+    public function setHiddenLabel($hiddenLabel) {
+        $this->_hiddenLabel = $hiddenLabel;
+    }
+
     /**
      * Sets the grouped elements and hides label
      *

lib/form/upgrade.txt

@@ -1,6 +1,10 @@
 This files describes API changes in core_form libraries and APIs,
 information provided here is intended especially for developers.
 
+=== 4.2.6 ===
+
+* New method `setHiddenLabel` added to the group form element type
+
 === 4.2.4 ===
 
 * The group element has a new method `getAttributesForFormElement` which should be used in conjunction