Added an example of combining QThread and QAbstractItemModel - a feat
that our customers ask every now and then
Task-number: QTBUG-135351
Pick-to: 6.8 6.9
Change-Id: I476e7121361e4d85c2d87eb663f0389a337a3085
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>