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.

data/settings.gd

@@ -79,6 +79,10 @@ var player_array: Array[PlayerObject]
 var edit_players: bool = false
 var currently_syncing: bool = false
 var character_select_open: bool = false
+## The ID of the game's play queue on each server, by server URL, set by
+## [code]MusicServerClient[/code]. Saved in the [code][queues][/code] section of
+## [method saved_settings_path], so the game goes on with the same queue after a restart.
+var queue_ids: Dictionary[String, String] = {}
 
 ## The game version, set in Project Settings > Application > Config > Version.
 ## Releases are made from it, see RELEASING.md.
@@ -153,6 +157,12 @@ func load_saved_settings() -> void:
 	for section: String in SAVED_SETTINGS:
 		for setting: StringName in SAVED_SETTINGS[section]:
 			set(setting, _saved_value(file, section, setting, get(setting)))
+	if file.has_section("queues"):
+		queue_ids.clear()
+		for server: String in file.get_section_keys("queues"):
+			var id: Variant = file.get_value("queues", server)
+			if id is String:
+				queue_ids[server] = id
 
 
 ## Saves the settings in [constant SAVED_SETTINGS], so they are the same the next time
@@ -164,11 +174,19 @@ func save_settings() -> void:
 	for section: String in SAVED_SETTINGS:
 		for setting: StringName in SAVED_SETTINGS[section]:
 			file.set_value(section, setting, get(setting))
+	for server: String in queue_ids:
+		file.set_value("queues", server, queue_ids[server])
 	var error: Error = file.save(path)
 	if error != OK:
 		GameLog.warning("Could not save the settings to %s: %s" % [path, error_string(error)])
 
 
+## Sets the play queue ID for [param server] and saves it.
+func set_queue_id(server: String, id: String) -> void:
+	queue_ids[server] = id
+	save_settings()
+
+
 ## Turns fullscreen on or off and saves it.
 func set_fullscreen(on: bool) -> void:
 	fullscreen = on