Work-around a bug in NVIDIA driver v344.11 affecting EVR-based renderers.

EVR, EVR-CP and Sync renderer would accept P010 pixel format when using the NVIDIA driver v344.11 even though this pixel format isn't supported by the EVR.
CPPageAccelTbl: Remove an unused function.
MakeAccelShortcutLabel: Skip non-defined hotkeys.

Fixes #4876.
Subtitle delay step tooltip: Show "<not defined>" when a hotkey isn't set.
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...