set 'enabled' key to a constant

As this key is reused in other bundle, it is safer to hold the key in a constant

Form/Type/Export/FilterType.php

@@ -38,6 +38,8 @@ class FilterType extends AbstractType
      */
     private $exportManager;
     
+    const ENABLED_FIELD = 'enabled';
+    
     public function __construct(ExportManager $exportManager)
     {
         $this->exportManager = $exportManager;
@@ -48,7 +50,7 @@ class FilterType extends AbstractType
         $filter = $this->exportManager->getFilter($options['filter_alias']);
         
         $builder
-                ->add('enabled', CheckboxType::class, array(
+                ->add(self::ENABLED_FIELD, CheckboxType::class, array(
                     'value' => true,
                     'data' => false,
                     'required' => false