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.
VSFilter: Use the same subtitle names as the ISR for external subtitles.

Use the video filename to be able to extract more information from the subtitle filenames.
Why "C:\tmp.srt"?
Fix some CPPCheck warnings or mark them as intentional.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...
Fix: x86 and x64 versions conflicted over file associations.

4f465cf8b81f1da800b1bfc57cc48e317d24fbb5 was incomplete for Vista+.

This commit fixes #4064.