More new room callbacks

[guwahavel]

The current POST_NEW_ROOM callback runs after all entities in a room layout have both been spawned AND have run their first update, which can lead to many inconveniences.

Variants of the NEW_ROOM callback which would be useful would be: -One which runs BEFORE any room entities are spawned -One which runs after all room entities are spawned BUT before any of them have run their first update (if possible)