Allow to set by default selected option(s) for views_handler_filter_in_operator

It is currently not possible to by default pre-select another option than either 'All' ('Optional' flag is checked) or the first option in the options list ("Force single" is checked) for an exposed filter of type views_handler_filter_in_operator.

I currently have overridden value_form in a subclass of views_handler_filter_in_operator to preselect the option we want (hardcoded, see code below) but it would be nice if a configurable (in the views GUI) default option (when "Force single" is checked), or multiple configurable default options would be available by default in views_handler_filter_in_operator.

 /**
   * Re-implementation of views_handler_filter_in_operator::value_form(), to
   * override the option selected by default
   */
  function value_form(&$form, &$form_state) {
    // parent's value_form alters form_state but we want to check the original form_state later on
    // so make a copy of it
    $orig_form_state = $form_state;
    parent::value_form(&$form, &$form_state);
    
    $default_value = 'mydefaultvalue';
    
    $form['value']['#default_value'] = $default_value;
    
    if (!empty($form_state['exposed'])) {
      $identifier = $this->options['expose']['identifier'];
      
      // original form state is checked here
      // as parent's value_form() already altered it to set the first option as the default
      if(!isset($orig_form_state['input'][$identifier])) {
        $form_state['input'][$identifier] = $default_value;
      }
    }
  }