Send a vu style signal to indicate audio activity per dongle to the frontend

[surfacescan]

This is arguably a feature that can be pushed off until later, except that when there is a conference call happening, the presenter has no way of knowing which caller is talking on which channel/dongle.

@miopa maybe you can brainstorm on a basic way, not a full vu-meter, to pass some visual indication that audio activity is happening on a given dongle. The indicator will then be shown in the calls tab of the voxbox