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.
CFile: Use the proper enum in Seek calls.
CTextFile: Make Seek more resilient by avoiding calling GetPosition/GetLength when possible.

This prevents ANSI subtitles files with Unix line endings to crash MPC-HC because of the standard library getting confused.

This commit fixes #4307.
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.