add subtitle default property

src/Abstract/Entity/HeaderBar.php

@@ -10,6 +10,7 @@ abstract class HeaderBar
 
     protected bool $_actions = true;
     protected string $_title = 'Yoda';
+    protected string $_subtitle = '';
 
     public function __construct()
     {
@@ -22,6 +23,10 @@ abstract class HeaderBar
         $this->gtk->set_title(
             $this->_title
         );
+
+        $this->gtk->set_subtitle(
+            $this->_subtitle
+        );
     }
 
     public function setTitle(