Use chance_flee for enemies

Closes #103

This allows for improved ranged enemy behavior. A roll of chance_flee
will determine if the enemy will attempt to move to a safe distance
(threat_range/2). Once the enemy has reached that distance, they will
not pursue the hero. When the hero gets close to the enemy, it will
either try moving to a safe distance again, or it will pursue the hero.
Don't pass input state to MenuItemStorage

This enables us to click/activate slots at a point other than the mouse
cursor. It will be useful for working with these slots on mouse-less
configurations.
Rename mouse parameter in menu functions to position
Update chance_flee comment in StatBlock
Merge pull request #684 from dorkster/input_state

Don't pass input state to MenuItemStorage
Merge pull request #678 from dorkster/chance_flee

Use chance_flee for enemies