Fix handling of changes to the root index of a VisualDataModel.

Fix handling of changes to the root index of a VisualDataModel.

Store the root index as a QPersistentModelIndex so the index remains
valid as the model changes, and in the case the root index is
removed from the model invalidate the contents of the VisualDataModel
until a new root index or model is set.

Change-Id: I1cbc27f2068f99a02ff3d43373905dec7e35e900
Reviewed-by: Martin Jones <martin.jones@nokia.com>