Internal filters page: Improve the context menu handling.

- The context menu can now be triggered using the keyboard.
- The context menu won't be quickly displayed before being replaced by the default context menu when right-clicking on the scroll bars.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
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...