This enables showing preview tooltip for all imported 3D models.
Also moved the preview image handling out of the model and into
NodeInstanceView, where it fits more naturally.
Change-Id: I48135d06aa8d9313525dae618e22692563da78fd
Fixes: QDS-2807
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
