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/unit/test_settings_files.gd

@@ -6,7 +6,7 @@ const SAVED_VALUES: Dictionary = {
 	&"auto_repeat_song": false, &"stop_after_current": false, &"hide_next_track": false,
 	&"hide_beginning": true, &"hide_for_seconds": 7.0, &"hide_length": true,
 	&"hide_ticks": true, &"winning_score": 13, &"inspiration_list_speed": 4,
-	&"fullscreen": true,
+	&"fullscreen": true, &"last_seen_version": "1.2.3",
 }
 
 var _snapshot: AutoloadSnapshot