ActionController: set a default window title

In case a controller action didn't set a view title this fix restores
the default title on XHR requests.

fixes #6797

library/Icinga/Web/Controller/ActionController.php

@@ -362,6 +362,8 @@ class ActionController extends Zend_Controller_Action
                 'X-Icinga-Title',
                 rawurlencode($this->view->title . ' :: Icinga Web')
             );
+        } else {
+            $resp->setHeader('X-Icinga-Title', rawurlencode('Icinga Web'));
         }
 
         if ($this->rerenderLayout) {