Open the About popup the first time a new version starts

The last version the About popup was shown for is saved as last_seen_version in
settings.cfg. When the game starts with another version, About opens to show
what is new.

tests/fakes/main_window_test_suite.gd

@@ -24,6 +24,8 @@ func before_test() -> void:
 	Settings.hide_next_track = true
 	Settings.use_low_played_mode = false
 	Settings.winning_score = 20
+	# Otherwise the About popup opens at start, as for a new version
+	Settings.last_seen_version = Settings.version
 
 	_server = FakeHttpTransport.new().install()
 	var mp3: PackedByteArray = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")