ISR/VSFilter: Support loading external PGS subtitles.

This commit fixes #1411.
GFN.cpp/.h: Rename the files to SubtitleHelpers.cpp/.h and cosmetics.

Add a new namespace, remove the external array, ...
Factorize the code used to guess the subtitle name and use it for external PGS subtitles.
Docs: Change the tag numbering of LAV Filters/ffmpeg.

Follow the same principle as the nightly builds numbering now that we use `git describe`.

MPC-HC 1.7.5 was a hotfix release so neither LAV Filters or ffmpeg were updated so we won't have any tag collision.