title: (symfony 1.4) Add an option in a Widget
body: <p>I have to add the option 'with_empty' like sfWidgetFormFilterInput in a sfWidgetFormDoctrineChoice. I know this option is not available with this widget but i have to add it, but i don't know how do to do it.</p>
  
  <pre><code>class TrainingFormFilter extends BaseTrainingFormFilter
  {
    public function configure()
    {
      $this-&gt;widgetSchema['tracks_list']
        -&gt;setOption('order_by', array('name', 'ASC'))
        -&gt;setOption('multiple', false)
        -&gt;setOption('add_empty', true);
    }
  }
  </code></pre>
  
  <p>Thanks.</p>