Screen.getHeight/Width used everywhere. GameEngine.update() is abstract.
tileSize replaced with new GameUnit. Using doubles for speed variables
to avoid rounding problems (e.g. speed becoming 0). Fix in Buttons:
button was not released when pointer moved.
Removed keycodes from buttons. Fixed Buttons not releasing. Zombie works
with intentions. Zombie can keep jumping, and will always try to go left
or right. Zombie.updateSprite() is based on movement, not state. Zombie
turns around when throwing object by touch. Replaced hardcoded speed
values. More gameUnits. Buttons are drawn at correct size. Buttons have
bigger touch area. If Zombie goes too slow, Guns get heat seeking
bullets. Bullet will not exceed range of weapon. Level empties lists in
clear(). Human has State.