Xpra: Ticket #2325: audio available signal

We detect silence and avoid sending empty packets since #1617, we could use similar code to start with audio disabled and notify the client when there is some audio to be played.

In particular, the html5 client. (#2322).

Sat, 23 Jan 2021 05:48:17 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2325