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.

tests/integration/test_stats_after_win.gd

@@ -12,7 +12,7 @@ func before_test() -> void:
 
 
 func _stats_checkbox() -> CheckButton:
-	return _quiz.get_node("QuizSettingsCard").get_node("%AddToDatabaseCheckButton")
+	return _window.get_node("SettingsPopup").get_node("%AddToDatabaseCheckButton")
 
 
 func test_winning_turns_statistics_off() -> void:
@@ -41,7 +41,7 @@ func test_statistics_turned_off_by_hand_stay_off_after_reset() -> void:
 func test_settings_checkbox_shows_statistics_off_after_win() -> void:
 	await _give_point_with_keybind(1)
 
-	_press("QuizSettingsButton")
+	(_window.get_node("SettingsPopup") as SettingsPopup).open()
 
 	assert_bool(_stats_checkbox().button_pressed).is_false()