Entities now spawn dynamically

Rather than being hard coded into being, I've started forward towards a
totally dynamic entity system, allowing a (technically infinite) amount
of entities to be included in the environment. These are not completely
dictated by map geometry, and can be spawned "on top" of the map. Soon,
I will include more entities such as boxes, and other such things. Right
now, they remain player characters.

For now, no noticeable change will occur in the engine, but this will
streamline things in the future.