TableView: expose rowAt method

Task-number: QTBUG-31160

Change-Id: I7dc6e0d755093786e079f526e5ff5f6cbcfa94e0
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>