Fixes the issue with image path to ‘add.png’ breaking when i18n site pre...

...site prefixes are in use. url() should not be used to get file paths. This is a documented issue, fix found here: https://api.drupal.org/comment/26869#comment-26869 . Note that the “better” solution in the comment following is not actually better - it does not work when Drupal is installed in a subdirectory and not in the server’s root.