* This allows us to add rich text support much more easily into other itemviews
like RDTableView.
* We set it up for debug messages so that resource links in debug messages can
be linked.
* We need to inherit to chain these two, as otherwise when the tree view
delegate calls sizeHint() it passes to the tree widget delegate, but
then has no way to return back to the tree view for the overridden
initStyleOption.
* If there was a built-in way to chain delegates like styles (which
solve this problem by calling back to baseStyle when going from one
function to another), or better yet a way to avoid the base
QStyledItemDelegate initStyleOption from completely trampling all over
the option passed in to any function, this wouldn't be necessary.
* 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.
* We add our own custom item model to handle the cases we need. We can
also make a few assumptions and optimisations around things we can
safely assume like e.g. nodel columns won't really change after init.
* This lets us have full control over batching updates, which was the
main motivation, but it makes it easier to extend in future (e.g.
adding single per-item tags instead of the heavyweight Qt::UserRole
data elements).
baldurk