Fix and Balancing

Thanks to AhDeh's feedbacks!

Building a new world: now it costs 500 for the number of world existing. So the first will be 500, the second 1000 and so on.

Trainers costs are updated: now the basic ones cost 1500 and the advanced 7500 to better balance the game.

You can change the screen mode (fullscreen or windowed) with the right ALT button instead of ENTER.

(Fix) Now when you create a new spawner the spawn frequency is correctly reporting the speed (instead of the one in the earlier spawner clicked).