Auto-load subtitles/audio: Improve multivolume rar file handling

Make it possible to load audio and subtitle files when playing
any .partXX.rar file of a multivolume rar archive:

Assume the following files:
movie.part01.rar
movie.part02.rar
movie.part03.rar
movie.srt
movie.mp3

Playing any of the three .rar files will now auto load the
mp3 and srt files.

Before, the .srt and .mp3 would have needed to be renamed to
something like
movie.part01.srt
movie.part01.mp3