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_settings_files.gd

@@ -95,6 +95,27 @@ func test_statistics_settings_are_not_saved() -> void:
 	assert_bool(file.has_section_key("quiz", "use_low_played_mode")).is_false()
 
 
+func test_queue_ids_are_saved_and_load_back() -> void:
+	Settings.set_queue_id("https://music.example.com", "queue-1")
+	Settings.winning_score = 12
+	Settings.save_settings()
+	Settings.queue_ids.clear()
+
+	Settings.load_saved_settings()
+
+	assert_str(_saved_file().get_value("queues", "https://music.example.com")).is_equal("queue-1")
+	assert_dict(Settings.queue_ids).is_equal({"https://music.example.com": "queue-1"})
+
+
+func test_file_without_queues_keeps_the_current_queue_ids() -> void:
+	Settings.save_settings()
+	Settings.queue_ids["https://music.example.com"] = "queue-1"
+
+	Settings.load_saved_settings()
+
+	assert_dict(Settings.queue_ids).is_equal({"https://music.example.com": "queue-1"})
+
+
 func test_fullscreen_is_saved_when_it_changes() -> void:
 	Settings.set_fullscreen(true)
 	assert_bool(_saved_file().get_value("window", "fullscreen")).is_true()