Add new language constant to all language files.

Signed-off-by: Thomas Hochstein <thh@inter.net>

lang/serendipity_lang_is.inc.php

@@ -1130,3 +1130,4 @@
 @define('MAINTENANCE_MODE_DEACTIVATE', 'Deactivate');
 @define('PLUGIN_LINK_SPARTACUS', 'More information');
 @define('PLUGIN_ALL_UPDATED', 'All Plugins updated');
+@define('MEDIA_DIRECTORY_MOVE', 'Move files to another directory');