Add support for game window notifications

src/game_base.h

@@ -285,6 +285,13 @@ void game_show_high_score(GameData *game, bool show_high_score);
 void game_show_lives(GameData *game, bool show_lives);
 void game_show_level(GameData *game, bool show_level);
 
+/*
+ * Sends a notification to the window associated with `game`.
+ *
+ * `message` - the notification message that will be displayed.
+ */
+void game_window_notify(const GameData *game, const char *message);
+
 /*
  * Updates game score.
  */