On call replaced callback

[eofster]

During standard call transfer, a call is being replaced with another one. For this event, PJSUA has a callback on_call_replaced. Apparently, this happens very rarely and never worked properly. With Asterisk for example, this doesn’t happen because Asterisk just reuses the same call when a transfer is made.

Currently, a transfer proceeds as expected, but the UI doesn’t reflect this. The call window shows the Gone status of the old call and disappears because the old call has ended. And the only way to end the new call is to quit the app.