title: Symfony2 how to include a parent formType into another
body: <p>i'm trieng to write a bundle for some fields.
  Now i got an class which i can extend.
  The class got some fields e.g. Title, Name etc.</p>
  
  <p>Now i want to write a formType which can be extended by another formtype.</p>
  
  <p>Like </p>
  
  <pre><code>class newsgroupType extends AbstractType
  {
  /**
   * @param FormBuilderInterface $builder
   * @param array $options
   */
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder
          -&gt;add('unusername','text',array(
                  'required' =&gt; true))
          -&gt;add('unactive','checkbox',array(
                  'required' =&gt; false));
  ...
  </code></pre>
  
  <p>The FormType which i want to extend is </p>
  
  <pre><code>class mainType extends AbstractType
  {
  /**
   * @param FormBuilderInterface $builder
   * @param array $options
   */
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder
          -&gt;add('unurl','text',array(
                'required' =&gt; false,
                'disabled' =&gt; true))
          -&gt;add('untitle','text',array(
                'required' =&gt; false))
  ...
  </code></pre>
  
  <p>Is there a way to extend the mainType to get all fields from it into my newsgroupType?</p>
  
  <p>Thanks a lot =)</p>