To allow for better performance when scaling to larger worlds we can't keep
iterating though all entities when checking visibility.
Instead we now have the PhysicalDomain keep track of all entities that are
either observering something, or being observed.
Since Bullet already contains efficient structures for doing space partioning and
collision detections we'll resuse that for visibility checks.
Each entity now has a sphere of "visibility" with which each perceptive entity does
checks against.
The Inventory domain is meant for all characters, i.e. those than can have
an "inventory".
It's a non physical domain, which means that entities contained in it won't
be affected by any physical constraints, or have physics applied to them.
Visibility for outside entities are by default forbidden unless an entity is
outfitted or wielded.
The idea is that other playes shouldn't see what's in the inventory of another
player, unless things are outfitted or wielded.
