Roundcube default listing mode plugin

listing_mode.php

<?php
class listing_mode extends rcube_plugin
{
  public $task = 'settings';

  function init()
  {
    $this->add_hook('preferences_list', array($this, 'preferences_list_hook'));
    $this->add_hook('preferences_save', array($this, 'preferences_save_hook'));
  }

  public function preferences_list_hook($args)
  {
    if ($args['section'] == 'mailbox') {
      $config = rcmail::get_instance()->config;
      $listview_select_order = new html_select(array('name' => '_default_list_mode', 'id' => '_default_list_mode'));
      $listview_select_order->add(rcube_label('list'), 'list');
      $listview_select_order->add(rcube_label('threads'), 'threads');
      $listview_options = array(
        'title' => $this->gettext('default_list_mode_label'),
        'content' => $listview_select_order->show($config->get('default_list_mode', 'list')),
      );
    
      $args['blocks']['main']['options']['listview'] = $listview_options;
    }

    return $args;
  }

  public function preferences_save_hook($args)  
  {
    if ($args['section'] == 'mailbox') {
      $args['prefs']['default_list_mode'] = isset($_POST['_default_list_mode']) ? $_POST['_default_list_mode'] : 'list';
    }
    
    return $args;
  }
}

Seems convenient, thanks for sharing! But.. can you explain where I need to place this file, and how to use it?