Save fullscreen and read server, password and debug from config.ini

The settings the game saves itself, the quiz settings and now fullscreen, move
to settings.cfg (settings_debug.cfg in debug mode).

config.ini in the same folder is edited by hand: the server to start with, the
API password and debug mode. The game writes it with everything commented out
when it is missing and never changes it after that. IniFile reads it: plain
key = value lines, whole-line # or ; comments and optional quotes.

tests/integration/test_quiz_settings_card.gd

@@ -174,6 +174,6 @@ func test_changes_are_saved_at_once() -> void:
 	_card.lower_winning_score_button.pressed.emit()
 
 	var saved: ConfigFile = ConfigFile.new()
-	assert_int(saved.load(Settings.quiz_settings_path())).is_equal(OK)
+	assert_int(saved.load(Settings.saved_settings_path())).is_equal(OK)
 	assert_bool(saved.get_value("quiz", "hide_next_track")).is_false()
 	assert_int(saved.get_value("quiz", "winning_score")).is_equal(19)

tests/integration/test_settings_popup.gd

@@ -35,10 +35,10 @@ 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()
+	Settings.save_settings()
 
 	var saved: ConfigFile = ConfigFile.new()
-	assert_int(saved.load(Settings.quiz_settings_path())).is_equal(OK)
+	assert_int(saved.load(Settings.saved_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()