changelog:
* when leaving the game from pause, Moneybags, Resourceful and Contender achievements update
* after you clear a wave and the upgrade screen pops up, the game is first slowed down and gradually comes to a stop after a few real-time seconds and ~0.5 in-game seconds instead of constantly going on in the background
* there is a tutorial pop-up when you play the game for the first time, which you can view again in pause