structure for enemy movement; extracted methods to simplify rendering

on player move now alerts nearby enemies that the player is moving, so
they may act in response; basic example added showing enemies that move
towards or away from the player based on their chance to win (their
chancetowin method returns from 0 - 50, 50 being a good enough chance to
win to go towards the player);

items are now rendered before (thus, appearing behind) enemies