Start using QObjectPrivate in private classes. Requires dependency to
core-private.

Task-number: QTRD-2819
Change-Id: I758783080de579f793457b50a1e32aeb5ab1a3eb
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>