Support Local Co-Op Players in the Online lobby

[RGonzalezTech]

Summary

I think it's really cool to do split-screen where the second player just has a (2) appended to the username. It makes these games more accessible and local co-op friendly.

Maybe the lobby system should support that, or I could have a branch dedicated to that implementation.

This would require some smarter spawning behavior, since the only identifiable information is the peer_id. We'll need to have an avatar_id or something instead since 1 peer_id can have multiple avatars or LobbyPlayer instances assigned to it.

Avatar ID

var avatar_id : String = "%d_%d" % [peer_id, local_idx] # 1_0 for host, 1_1 for host's guest player

Then, we update all peer_id references to use this new avatar_id instead. Sounds simple... on paper, lol.