MediaInfo tab: Enable the "Save As" button only if some info is available.

Being that the tab can be displayed before the info is available or even if none could be found, it makes sense to disable the button in those cases.
DSUtil: Remove duplicated function "FileExists".
Cosmetics: Remove unused include.
DSUtil: Remove duplicated function GetProgramPath.
Cosmetics: Use PathUtils::GetProgramPath when possible.
VSFilter: Remove an unused function.
Cosmetics: External toolbar image: Replace nested if by loops.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
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...