Too much layers of complexity. Instead of keeping an eye on
the "live" tree model and switch to a static StandardItemModel
whenever live synchronization is not possible (and do all the
tree view snake oil magic that's needed to make the switch
appear "smooth") keep static copies of relevant parts of the
live model, and update them whenever the the live model changes.

Change-Id: I88a7de67f7703cd2fed041351346b1c7ada0839e
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: hjk <hjk@theqtcompany.com>