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_settings_popup.gd

@@ -0,0 +1,55 @@
+extends MainWindowTestSuite
+## The statistics settings in the application Settings popup.
+
+var _popup: SettingsPopup
+
+
+func before_test() -> void:
+	super()
+	_popup = _window.get_node("SettingsPopup")
+
+
+func test_less_played_toggle_changes_the_setting() -> void:
+	Settings.use_low_played_mode = false
+	_popup.open()
+	await _runner.simulate_frames(1)
+	assert_bool(_popup.low_played_button.button_pressed).is_false()
+
+	await _click_at(_popup.low_played_button.get_global_rect().get_center())
+
+	assert_bool(Settings.use_low_played_mode).is_true()
+
+
+func test_statistics_toggle_changes_the_setting() -> void:
+	Settings.add_to_stats = true
+	_popup.open()
+	await _runner.simulate_frames(1)
+	assert_bool(_popup.add_to_database_button.button_pressed).is_true()
+
+	await _click_at(_popup.add_to_database_button.get_global_rect().get_center())
+
+	assert_bool(Settings.add_to_stats).is_false()
+
+
+func test_statistics_settings_are_not_saved_with_the_quiz_settings() -> void:
+	Settings.add_to_stats = false
+	Settings.use_low_played_mode = true
+
+	Settings.save_quiz_settings()
+
+	var saved: ConfigFile = ConfigFile.new()
+	assert_int(saved.load(Settings.quiz_settings_path())).is_equal(OK)
+	assert_bool(saved.has_section_key("quiz", "add_to_stats")).is_false()
+	assert_bool(saved.has_section_key("quiz", "use_low_played_mode")).is_false()
+
+
+func test_popup_fits_its_rows_and_is_centred() -> void:
+	_popup.open()
+	await _runner.simulate_frames(1)
+
+	var panel: Rect2 = _popup.settings_panel.get_global_rect()
+	var rows: Control = _popup.settings_panel.get_node("VBoxContainer")
+	assert_bool(panel.encloses(rows.get_global_rect())).is_true()
+	# The rows' own height fits, so the last toggle is not squeezed or cut off
+	assert_float(rows.get_combined_minimum_size().y).is_less_equal(rows.size.y)
+	assert_vector(panel.get_center()).is_equal(Vector2(960, 540))