ENH: Re-implemented vtkGarbageCollector.  The new version has a much simpler interface.  Classes now need only a ReportReferences method.  The RemoveReferences method is no longer needed.  A new reference graph analysis engine has been implemented that reduces redundant walks during a single deletion.  It now also supports delayed collection (though no interface yet exists to enable it).  Delayed collection will accumulate UnRegister calls without doing a full collection check.  The next full check accounts for the accumulated UnRegister calls.  This feature will be used to reduce the overhead of garbage collection.