Auto-change fullscreen monitor mode: Make the settings handling sane.

The settings weren't even initialized on first run. Also simplify a lot the GUI code used to modify those settings.

This commit fixes some cases where the auto-change fullscreen mode monitor settings got corrupted or missing.

Fixes #4298.
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.