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.

data/song_object.gd

@@ -12,6 +12,10 @@ var players_given_point: Array[String] = []
 var is_answered: bool = false
 var has_played: bool = false
 var is_playing: bool = false
+## The song's file in [method Playlist.songs_folder]. Read the song with
+## [method Playlist.get_song].
+var song_path: String = ""
+## The song itself, only when it could not be saved to a file.
 var song: AudioStream