Fix crash when an QuickItem's parent is set to 0.

The itemreparented-slot now checks if the parent was set to 0
(=> removed from the scene, without deleting) and in this case removes
the item from the model.
QuickItemModel: Remove children of removed items.

This commit fixes a crash when accessing the children of an item whose
parent has been removed from the model. Until now the children
remained in the model's internal data set, now they're removed
correctly, when the parent is removed.