* This item delegate will forward on either to a specified other
delegate or to the base implementation. This allows chaining delegates
(i.e. having one built-in to the widget, which forwards to a user-set
delegate).
* There's a messy order-of-operations thing to deal with various
different themes, as well as a couple of minor problems, but at least
the highlight and selection bars draw fairly consistently and don't
leave an ugly gap over the branches.
baldurk