MDL-78163 lib: Added class properties that are not declared in form

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

lib/form/button.php

@@ -46,6 +46,9 @@ class MoodleQuickForm_button extends HTML_QuickForm_button implements templatabl
     /** @var string html for help button, if empty then no help */
     var $_helpbutton='';
 
+    /** @var bool if true label will be hidden. */
+    protected $_hiddenLabel = false;
+
     /**
      * constructor
      *
@@ -89,4 +92,13 @@ class MoodleQuickForm_button extends HTML_QuickForm_button implements templatabl
             return 'default';
         }
     }
+
+    /**
+     * Sets label to be hidden
+     *
+     * @param bool $hiddenLabel sets if label should be hidden
+     */
+    public function setHiddenLabel($hiddenLabel) {
+        $this->_hiddenLabel = $hiddenLabel;
+    }
 }