New Model structure

[Eric Dean Tribble <tribble>]

hasDependents() - return true if there are any dependents.

   Why is this message needed?

Optimization.  So the model doesn't have to create an update record
unless it actually has to broadcast changes.  We won't use this
immediately, but it will likely be important for heavily used model
types like Tables.

   Necromancers should go away and have their functionality subsumed by
   the dependents mechanism.  This will make the code much less
   depressing. 

good

      For the mechanics out there, should the dependents be a set or a bag?

   A Set.  (Am I a mechanic?)

Yes.  Why should it be a set?  (I'm neutral, but I want a reason to go
one way or the other).

dean