Replaces double to float in Geometry::Circle.
Adds name() to both Sprite::Thing and World::Actor.

This will yield the name of the object, either given by the JSON
description or the base filename.
Adds collideWithEvent() to World::CollisionObject.

This method will return the id of the event generated by colliding with
the given object. I may want to add some information about which tile or
map has been collided with (maybe tile type?) by appending it to the
string in a predictable manner.
Appends type of collision object to the event string.

I don't want there to be collisions (heh) in the naming of actors. For
instance, a 'map' actor would have the same event name as colliding with
the map itself. That would be bad.