First of all it must be a QGraphicsItem, because:

Qt Code: 
  1. QGraphicsScene::addItem ( QGraphicsItem * item )
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QGraphicsLayout does not seem to be a subclass of QGraphicsItem.

Although I less and less see what would such grid be useful for and how would it differ from QGraphicsGridLayout.
It's about the "little extra" I haven't dived into.

So, can anyone make a real,feasible suggestion, without throwing with classnames at me, please?