added support for the sitemapnavi plugin

When installed the navigation can be switched between the default
sidebar based navigation and the site tree

lang/en/lang.php

@@ -59,6 +59,9 @@ $lang['image_detail']               = 'image details';
 $lang['mode_edit']                  = 'Editing Page';
 
 
+$lang['nav_sidebar']                = 'Navigation';
+$lang['nav_sitemap']                = 'Site Tree';
+
 /**
  * preferences colors
  */