Help Requested: INotifyActorMovement

I need help with the Lua tie-in and testing. I've haven't had much time to spend on this over the past three weeks, and rather than let it lie stagnant, I figured asking for help.

Closes  #6204. (Or meant to)

Pinging @obrakmann as he knows a bit about this.

(Sidenote: I've attempted to use this same code for shroud and crushing, but once you have 300+ listeners + movers the lag becomes visible, so I've restricted this to region checking for now.)
I think this is better - the `@` is entirely for YAML-parsing purposes, and the name is thrown away pretty early on.