Remove currentRowItem and rename positionViewAtIndex

Again trying to make the api a bit more consistent at using
row vs. index.

Change-Id: I769ca34c7bde628757a19fd47744b5cb1054a154
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>