playlist: sort title and album by filename order
When sorting a playlist by title or album, use the filename sort order (i.e. 2-title should come before 10-title). strcasecmp would previously sort 10-title to come before 2-title. strverscmp would also solve the issue but we use vlc_filenamecmp for the increased accuracy as vlc_filenamecmp takes user locale into consideration.
Use strverscmp for the sort order of URLs, as URLs are generally in ASCII and not locale-dependent.