fancy new FieldOfView 

it remembers what has been seen
have to construct it in PlayScreen
pass it to creature factory
pass it to playerai

displayupdate calls fov.update to update remembered map state

bug: everything is grey right now, fungi are not displayed
fix vision colour and vision of creatures

mostly just rewrote displayTiles() in PlayScreen

added a getter to creatures list in World

fiddled with other stuff