* Annoyingly although Qt has an internal bool allowUserMoveOfSection0
which does exactly what we want allowing the tree column to be
movable, we can't enable it because it's private. So instead we have
to re-implement section moving ourselves.
* Minor tweak - also made RDTreeWidgets non-movable by default since
usually we don't want to allow it.
* 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.
* This still isn't perfect with some custom style stuff but it's next
to impossible to get working right. In theory we should manually
draw PE_PanelItemViewItem in the gap in drawRow() or so to make sure
we also draw any on-hover highlight over objects, but that doesn't
seem to work.
* 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