changed: Add logic to handle subtitles for stacked files

Subtitles for stacked items require additional work besides this but at least one can download subtitles again for each stack item separately. In order to improve this further there are 3 options (I think):
- Assume subtitle services provide zip's for multipart (aka stacked) movies and act accordingly;
- Show a dialog for each stack item;
- Automatically try obtain additional subtitles according to the volume name (eg. for .cd1.srt, also try to download .cd2.srt)

Not sure what the best approach is. @amet / @jmarshallnz: Your thoughts?