attempt to work around engine player model bug

attempt to work around engine player model bug

when the world loads and the player spawns in, or is
teleported to the same node space as a sign, occasionally, the
player's model goes flat and their skin gets changed to the
whatever the sign's text is.

Also, when checking if an entity needs spawned, or when
deleting, try to make absolutely sure that only sign-related
entities will be selected for re-use, or selected for deletion
(we don't want some player's prize cow to be deleted)

I think this will fix it, or at least it'll surely prevent
signs_lib from being able to cause it.

Also, moved another function to the entity-handling section of
the file and made it global (signs_lib namespace) in the
process.