Send the play queue ID to the server

The server keeps a play queue per client, identified by the X-Queue-Id
header. The client sends the ID it has for the selected server with every
request and keeps the one in each response, which is new after a reset or a
sync. The IDs are saved per server in the [queues] section of settings.cfg,
so the game goes on with the same queue after a restart.

HttpTransport now passes the response headers on to the client.

tests/unit/test_http_transport.gd

@@ -32,7 +32,7 @@ func test_request_to_silent_server_times_out_as_failed() -> void:
 
 	transport.request("http://127.0.0.1:%d/music/rand" % _server.get_local_port(),
 			HTTPClient.METHOD_GET,
-			func(response_code: int, _body: PackedByteArray) -> void:
+			func(response_code: int, _body: PackedByteArray, _headers: PackedStringArray) -> void:
 				response_codes.append(response_code))
 	for i: int in 40:
 		_accept_connections()