Add Clear cache to the settings

A new row, "Downloaded pictures and songs", shows how much space the cached
character pictures, covers and downloaded songs take, with a Clear cache
button. Clearing deletes them, but keeps the songs the current quiz uses,
the settings, config.ini and the saved quiz. The characters are loaded
again at once; covers come back when they are shown.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

data/playlist.gd

@@ -175,6 +175,15 @@ func store_song(song_object: SongObject, mp3: PackedByteArray) -> void:
 
 
 ## Deletes the files of the songs downloaded so far, except those in [param keep].
+## The downloaded song files the playlist uses, which must be kept while it does.
+func song_files_in_use() -> PackedStringArray:
+	var files: PackedStringArray = []
+	for song_object: SongObject in song_object_array:
+		if !song_object.song_path.is_empty():
+			files.append(song_object.song_path)
+	return files
+
+
 func clear_song_files(keep: PackedStringArray = []) -> void:
 	if !DirAccess.dir_exists_absolute(songs_folder()):
 		return