Property dialog: Do the MediaInfo analysis asynchronously.

This avoids having to wait for the property dialog to open because of an info some people might not want to look at.
LAV Filters: Distribute CHANGELOG.txt, README.txt, COPYING files (PoC).
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.