Adds a OfflineMultiplayerPeer class which behaves like a server with no
connected peers.
Use OfflineMultiplayerPeer as default for SceneMultiplayer.
This means that the SceneTree will act as the multiplayer authority by
default.
Calls to is_server will return true, and calls to get_unique_id will
return TARGET_PEER_SERVER.
Fabio Alessandrelli