QGraphicsItem::setLayout assumes that the layout is not a QGraphicsItem

QGraphicsItem::setLayout ends up reparenting (setParentItem()) all the items in the layout.
Instead it should first call layout->graphicsItem(). If that returns true then it should just reparent that, leaving the items inside the layout alone.

This allows for layouts to be hidden, and for layouts to do more fancy things (for example, to hide some of its items). This is essential for animated layouts in libdui.