Refactor to fit game-shell structure; externalized lobby client code

scripts/client_networking.gd

@@ -54,6 +54,10 @@ func close_connection() -> void:
 
 func unset_client() -> void:
 	if not peer: return
+	if not multiplayer:
+		peer.close()
+		peer = null
+		return
 
 	multiplayer.connected_to_server.disconnect(client_connected_to_server)
 	multiplayer.connection_failed.disconnect(client_connection_failed_to_server)