XySubFilter support

Adds support for the subtitle interface used by XySubFilter. Instead of auto-loading this PR offers the subtitles on the graph when "auto-load subtitles" is turned on but the ISR is disabled. (ISR is always disabled atm until a seperate option is introduced)


To-do list:
- [ ] Support more external subtitle formats in SubtitleSource.
- [ ] Introduce an option to disable the ISR without disabling auto-loading subtitles.