Fix positioners to apply a "populate" transition for initially added
items. This is consistent with ListView and GridView and also fixes the
behaviour from QtQuick 1.x where the positioners were instead running
the "move" transition for initially added items.

Change-Id: Ib43f1141ce3e7379df085c178b684f89b8567403
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>