RFC: Simplify even more the use of wertcsink + webrtcsink

After a couple of days working with webrtcsink and webrtcsrc I think its usage could be simplified even more for the use case of a single producer and a single consumer.

Through properties, webrtcsink could automatically start the signaling server and webrtcsrc could auto-connect to the first peer. This would allow faster iterations of debugging without looking up server logs to identify the peer-id.

For the sink

webrtcsink start-signalling-server=true signalling-server-path=~/path/to/signalling-server

For the source it could be done in several ways.

webrtssrc peer-index=0
webrtssrc auto-connect-to-first-producer=True