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.
Fix some CPPCheck warnings or mark them as intentional.
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.
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.