Option/scriptable that ghosts can pass walls and doors

It would be nice if it was possible to make creatures/NPCs pass walls and other objects.

So collision should be turned off. Whatsoever it would be nice if it could be set/unset that the ghosts stayy above a surface - most interesting is the ground here.

(RM-2681 from redmine: created on 2015-06-14 by Who Knows, )