Save the quiz settings to the config folder

The quiz card's settings are saved to quiz_settings.cfg in OS.get_config_dir()
on every change and loaded at start. Debug builds use their own file, and
MUSICPLAYER_CONFIG_DIR moves the file elsewhere so tests never touch it.

Count plays in statistics and Prefer less played soundtracks are not saved and
move from the quiz card to the Settings popup.

tools/take_screenshots.gd

@@ -2,7 +2,10 @@ extends SceneTree
 ## Takes a screenshot of the main window in every UiTour state, against a fake server.
 ##
 ## Usage:
-##   godot --path . -s tools/take_screenshots.gd -- <output dir>
+##   MUSICPLAYER_CONFIG_DIR=$(mktemp -d) godot --path . -s tools/take_screenshots.gd -- <dir>
+##
+## Setting [code]MUSICPLAYER_CONFIG_DIR[/code] keeps your saved quiz settings out of the
+## pictures.
 ##
 ## Writes [code]<state>.png[/code] for each state and [code]layout.json[/code] with where
 ## every node is in each state. With [code]--headless[/code] only the layout is written.