PGS/DVB subtitles: Simplify the code.

Use only 3 classes instead of 4. Will be useful for some future work.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
Fix some CPPCheck warnings or mark them as intentional.
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.