Gave up on using PreDrawHalos, gmod_tool is too messed up

For some reason, the way the gmod_tool was coded causes tool functions
like TOOL:Init() to be called on EVERY client when ANY client respawns.
It pretty much made using halos in PreDrawHalos unreliable.

Instead, I've unfortunately moved the halos back into TOOL:Think(),
which means there will be a large FPS drop when using halos. Sorry :/

Added stacker_stayinworld and stacker_set_stayinworld for server owners
that want to restrict stacker so that props MUST be spawned inside of
the world. Apparently some DarkRP server(s) crash(es) when many props
are spawned in the world and then are unfrozen.