Threat system, smarter targeting

[BillyWM]

Targeting is currently dumb - picks first enemy it can find. This often is one way off the screen - will walk to enemy, leaving itself open to attack from other one.

Calculate a threat value based on multiple weighted factors:

• Distance from player
• Whether enemy is holding item (haven't found memory addr for this)
• Whether enemy attacked recently (aggressiveness)
• Kind of item held (Maybe. Not terribly important)