Using the MouseArea3D linked to gizmo to manage the drag leads to
slight unwanted drift on the drag plane, as it moves with the gizmo.
Fixed by using a helper MouseArea3D to calculate drag vector.
Helper area's transform is set at mouse press time and is not changed
during the drag.
Change-Id: I50d3f738277bd34005c977423d114abc8dbed520
Fixes: QDS-1312
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>