[reSIProcate] DUM shutdown message processing

[Derek MacDonald]

> Another problem, if application allocates an
> ApplDialogSet on heap, and forgets to delete it, the
> DUM::shutdown() will never be called. In this way, the
> application will hangup there.
> 

DUM will delete AppDialogSets(that are associated with a DialogSet) unless
you overload the AppDialogSet::destroy method.

If you are creating an AppDialogSet out of context, you have to clean it up
yourself, but this isn't how AppDialogSet is meant to be used.

--Derek