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.

tests/unit/test_song_files.gd

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+extends GdUnitTestSuite
+## Downloaded songs are kept in files in the songs folder, not in memory.
+
+var _snapshot: AutoloadSnapshot
+var _config_dir_before: String
+var _mp3: PackedByteArray
+
+
+func before_test() -> void:
+	_snapshot = AutoloadSnapshot.take([Playlist])
+	_config_dir_before = OS.get_environment(Settings.CONFIG_DIR_ENV)
+	OS.set_environment(Settings.CONFIG_DIR_ENV, create_temp_dir("song_files"))
+	Playlist.clear_song_files()
+	Playlist.song_object_array.clear()
+	_mp3 = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")
+
+
+func after_test() -> void:
+	Playlist.clear_song_files()
+	OS.set_environment(Settings.CONFIG_DIR_ENV, _config_dir_before)
+	_snapshot.restore()
+
+
+func _add_song() -> SongObject:
+	var song_object: SongObject = SongObject.new()
+	Playlist.store_song(song_object, _mp3)
+	Playlist.add_to_playlist(song_object)
+	return song_object
+
+
+func test_stored_song_is_a_file_and_plays_back() -> void:
+	var song_object: SongObject = _add_song()
+
+	assert_object(song_object.song).is_null()
+	assert_bool(FileAccess.file_exists(song_object.song_path)).is_true()
+	var stream: AudioStream = Playlist.get_song(0)
+	assert_object(stream).is_instanceof(AudioStreamMP3)
+	assert_float(stream.get_length()).is_greater(0.0)
+
+
+func test_every_song_gets_a_file_of_its_own() -> void:
+	var first: SongObject = _add_song()
+	var second: SongObject = _add_song()
+
+	assert_str(first.song_path).is_not_equal(second.song_path)
+	assert_array(DirAccess.get_files_at(Playlist.songs_folder())).has_size(2)
+
+
+func test_downloading_a_song_again_replaces_its_file() -> void:
+	var song_object: SongObject = _add_song()
+	var old_path: String = song_object.song_path
+
+	Playlist.add_song_to_song_object(0, _mp3)
+
+	assert_str(song_object.song_path).is_not_equal(old_path)
+	assert_object(Playlist.get_song(0)).is_not_null()
+
+
+func test_resetting_the_playlist_deletes_the_files() -> void:
+	_add_song()
+	_add_song()
+
+	Playlist.reset_playlist()
+
+	assert_array(DirAccess.get_files_at(Playlist.songs_folder())).is_empty()
+
+
+func test_missing_file_gives_no_song_so_it_is_downloaded_again() -> void:
+	var song_object: SongObject = _add_song()
+
+	DirAccess.remove_absolute(song_object.song_path)
+
+	assert_object(Playlist.get_song(0)).is_null()
+
+
+func test_song_stays_in_memory_when_it_cannot_be_saved() -> void:
+	# A file where the folder should be, so the folder can't be made
+	var blocker: String = create_temp_dir("song_files_blocked").path_join("not_a_folder")
+	FileAccess.open(blocker, FileAccess.WRITE).store_string("x")
+	OS.set_environment(Settings.CONFIG_DIR_ENV, blocker)
+
+	var song_object: SongObject = _add_song()
+
+	assert_str(song_object.song_path).is_empty()
+	assert_object(Playlist.get_song(0)).is_instanceof(AudioStreamMP3)