* 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