Removed the global actors list.
Keep a global list of objects in Core (accessible from outside, for now
at least).
Create a room local list of actors.
Introduced Actor::SetArea() and Area() (not used yet)
Adapted code.
Moved UpdateSee back to Actor
CreateCreature: if point is -1, -1 add the creature near the main player.
Introduced Room::ActorEnteredArea/ActorExitedArea.
Fix actor deletion.