Move menu handling to its own class.
Navigating through the menu is now possible with arrow keys too and pressing enter will complete the action of the selected option.
Enemies will now start moving in the other direction when they hit a gameobject wall. Additionally, bullets are drawn again and get rid of a lot of unused, commented out and/or deprecated code related to bullets.
Bullets are properly drawn with their image on the screen again, enemies and players are now able to die (and bullets kill them). Corrected positions and waypoints of enemies (three in total). Remove some useless, commented our and/or unused code.
Bullets are now drawn but without moving when the game is paused.
From now on bullets, enemies and players are all drawn with a ~50% opacity decrease when the game is paused. This was already done for tiles.