Save the quiz and continue it after a restart; load characters from a folder

The quiz is saved to current_quiz.json in the config folder after every
change, and the game goes on where it was when it starts again: the
players with their points and characters, every song with who got a point
for it and whether it was played, answered or is playing, and the songs
downloaded ahead with their files. The song that was playing is ready to
play again. Songs whose files are gone are downloaded again.

On a server, the saved songs are checked against the server's playlist. If
it changed, e.g. after a sync, or another server is used, the quiz starts
over with the same players. A quiz from a music folder continues offline,
including how far a prepared quiz has come. A new game is saved like any
other change, and a file that can't be read starts a new quiz.

Characters can be read from a folder picked in Settings instead of the
server, for this session. A music folder with a characters folder brings
its own characters, e.g. for a themed quiz; that folder is not counted as
a soundtrack. When the characters come from somewhere else, players pick
their characters again.

Closes #11, closes #44

data/player_object.gd

@@ -21,6 +21,9 @@ var id: String = _generate_uuid()
 
 var is_first_point: bool = true
 var character: Texture = load(NO_CHARACTER_PATH)
+## The name of the picked character, its picture's file name, or empty when none is
+## picked. Saved with the quiz, so the picture can be found again after a restart.
+var character_name: String = ""
 
 
 func _init(new_name: String) -> void:
@@ -28,6 +31,39 @@ func _init(new_name: String) -> void:
 	player_score = 0
 
 
+## Shows [param texture], the character called [param new_name], for the player.
+func set_character(texture: Texture, new_name: String) -> void:
+	character = texture
+	character_name = new_name
+
+
+## Goes back to no character, e.g. when the characters come from somewhere else.
+func clear_character() -> void:
+	character = load(NO_CHARACTER_PATH)
+	character_name = ""
+
+
+## The player as a Dictionary that can be written as JSON, for [QuizSave].
+func to_dict() -> Dictionary:
+	return {
+		"id": id, "name": player_name, "score": player_score,
+		"is_first_point": is_first_point, "character": character_name,
+	}
+
+
+## A player made from [param data], as given by [method to_dict], or null when
+## [param data] is not a player. The character's picture is not loaded here.
+static func from_dict(data: Variant) -> PlayerObject:
+	if !(data is Dictionary) || !(data.get("name") is String) || !(data.get("id") is String):
+		return null
+	var player: PlayerObject = PlayerObject.new(data.name)
+	player.id = data.id
+	player.player_score = maxi(int(data.get("score", 0)), 0)
+	player.is_first_point = bool(data.get("is_first_point", player.player_score == 0))
+	player.character_name = str(data.get("character", ""))
+	return player
+
+
 ## Returns a random version 4 UUID, e.g. "3f2b8c1e-9a4d-4e7f-b2c6-5d8e1f0a7b93".
 static func _generate_uuid() -> String:
 	var bytes: PackedByteArray = Crypto.new().generate_random_bytes(16)