This is needed for subsequent feature work which needs to be able
to turn this off.
This review also includes a number of bug fixes to the model:
- Fix an off-by-one when explicitly breaking a group apart.
- Fix rowCount() returning the value for a top-level item with the same row when called for a group member.
- Fix updating/invalidating persistent indices endRemoveRows() doesn't know how to handle.
- Update data for former parent item ahead of child-reparenting insert when breaking a group apart.
None of these fixes actually matter for the applet view, but
they obviously improve correctness, and will matter for later
work.