Set image drag hotspot from drag image origin.

QtWebKit does not currently set the drag image origin, this cause
any dragged image to always visibly jump as soon as the drag start.

By setting the hotspot any dragged image will start at its origin and
only move when then the mouse moves.

Change-Id: I6cdfa1bdbcf79a63f26fc62442978f65aabd493b
Reviewed-by: Michael Bruning <michael.bruning@digia.com>