Partway there.
Move can_see from actor to unit.

Only units will need to see things, right?
Maybe guided missiles, but if so then we can make the
change.
Change styling in wall.corners
Ensure that walls are added to world.walls
Finish debugging vision
Add visibility checking to event throwing.
Integrate vision with existing event systems
Ensure vision events get sent
Fix hidden unit rendering
Merge branch 'feature-occlusion' into develop

Conflicts:
	event.py
	world.py