webrtc: janusvrwebrtcsink: add 'use-string-ids' property

Instead of exposing all ids properties as strings, we now have two signaller implementations exposing those properties using their actual type. This API is more natural and save the element and application conversions when using numerical ids (Janus's default).

I also removed the 'joined-id' property as it's actually the same id as 'feed-id'. I think it would be better to have a 'janus-state' property or something like that for applications willing to know when the room has been joined. This id is also no longer generated by the element by default, as Janus will take care of generating one if not provided.