Self == Ent case for entity_entered_map
start_link used by map when spawning entities
Updating vis position on self entered map
entity_mgr:start() for debugging scenarios
Using add_sup_handler for gen_event components

It might turn out that some custom (entity, component) behaviour will be
needed, for now gen_event with add_sup_handler should suffice
Multiple entity_entered_map case

Don't like the way this module complexity grows, it could use some
abstraction over those gb_trees maybe