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>

common/cover_cache.gd

@@ -38,6 +38,11 @@ static func folder() -> String:
 	return Settings.config_dir().path_join(FOLDER_NAME)
 
 
+## Deletes every cached cover. They are downloaded again when next asked for.
+func clear() -> void:
+	_files.clear()
+
+
 ## Deletes the covers of soundtracks that are not in [param covers] (UUID -> cover
 ## time), and those whose soundtrack no longer has a cover.
 func keep_only(covers: Dictionary) -> void: