This allows for more use cases to be handled like having several session
between 2 peers, and it simplifies the code a bit and makes the protocol
sensibly cleaner
Webrtcsink has been refactored a bit to take the new concept into
account
In some applications people might need to be able to pass more metadata
than the only "display-name" this commit allow that by adding a field in
all signalling structure where it is free form json and removing the
"display-name" as the free form `meta` field allows application to pass
this kind of information without any problem.
On the "webrtcsink" side this commit adds a `meta` property as a
gst::Structure which will be passed as a json string to the signalling
server so we enforce data to be structured.
