Add TabView::contentItem

Task-number: QTBUG-38425
Change-Id: I9e2b960e9b336145388371e5cee8a8a4cb1fa38e
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>