Option to adjust the volume of contacts in a call

In single calls: adjust the volume of the contact In conference calls: adjust the volume for each participant separately

Adjusting the squelch would also be very useful, especially for #855 (closed) ("Confused audio in multiparty video conference - please add control/audio allocation/handsup flag"). It would also help a bit with the problems behind #822 (closed) (push-to-talk and push-to-mute).

mentioned in issue #1276 (closed)