Allows to hide a field during an AJAX context

When request is an AJAX request (with modals), I've added a `ajax_hidden` option to hide fields.

Here are the use cases:

| Option value | Field displayed in AJAX? |
| ---------------- |---------------------------------|
| not filled        | Yes                                 |
| false              | Yes                                 |
| true               | No                                   |

Here is how to use:

```php
/**
 * {@inheritdoc}
 */
protected function configureListFields(ListMapper $listMapper)
{
    $listMapper
        ->add('myfield', 'string', array('ajax_hidden' => true))
    ;
}
```