[Laszlo-dev] classChildren should be a `static var children`

[P T Withington]

I'd like to make classChildren be a class var (and just name it  
children), instead of sticking it in the class prototype the way we do  
now.  I think this makes it more parallel to the setters, etc.  Like  
the setters, when you instantiate a node, you append any children of  
the instance to the class children to get all the children of the  
instance.  If we always define it, we can clean up a bunch of the  
silly code that has to check for it existing and just say:

   this.children = this.constructor.children.concat(children);

Sound ok?