DVB: Lock the player when the scan dialog is opened.

The DVB scan can take quite some time and we don't want it to be interrupted by a careless double-click on a media file. It also avoids all issues related with destroying the filter graph while scanning.

Fixes #5130.
(WIP/Broken) Disable the time tooltip shadow for incompatible video renderers.

Some video renderers cause the tooltip shadow to be replaced by an opaque black box.

Fixes #1415.
(WIP/Broken) Try to find out if VMR-7 uses overlays, without luck so far...
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.
Capture bar: Try to guess what was intended here.

So far getting the sign of the original biHeight seems to be what makes more sense. Some formats do use negative biHeight so it seems sensible to keep the original sign.