Cache character pictures and songs on disk

Character pictures are kept in characters/ next to the settings, with an
index.json of when the server last changed each one and when it was
downloaded. Only new or changed pictures are downloaded, using the server's new
/character/details. Older servers fall back to the name list, and pictures are
downloaded again after 7 days.

Downloaded songs are written to songs/ and only their paths are kept in memory.
The folder is emptied when the game starts and when the playlist resets.

popups/character_card.gd

@@ -16,17 +16,15 @@ func _ready() -> void:
 	mouse_exited.connect(_set_hovered.bind(false))
 
 
-## Shows [param character_name] and fetches its picture from the server.
-func set_character_name(character_name: String) -> void:
+## Shows [param character_name] and its picture. [param fetch_image] is called with the
+## name and a Callable that takes the picture as an [Image], e.g.
+## [method CharacterCache.fetch_image].
+func set_character_name(character_name: String, fetch_image: Callable) -> void:
 	character_name_label.text = display_name(character_name)
 	tooltip_text = character_name_label.text
-	var image_fetched: Callable = func image_fetched(image: PackedByteArray) -> void:
-		var picture: Image = image_from_bytes(image)
-		if picture == null:
-			GameLog.warning("Could not load the picture of ", character_name)
-			return
+	var image_fetched: Callable = func image_fetched(picture: Image) -> void:
 		character_image.texture = ImageTexture.create_from_image(picture)
-	MusicServerClient.fetch_character_image(character_name, image_fetched)
+	fetch_image.call(character_name, image_fetched)
 
 
 ## The name to show for a picture's [param file_name]: without the file extension, and