Form submission handler for the 'preview' action.
$form: An associative array containing the structure of the form.
$form_state: A reference to a keyed array containing the current state of the form.
public function preview(array $form, array &$form_state) {
$comment = $this
->getEntity($form_state);
drupal_set_title(t('Preview comment'), PASS_THROUGH);
$form_state['comment_preview'] = comment_preview($comment);
$form_state['rebuild'] = TRUE;
}