Maintain referential integrity of shared QNode resources

When a QNode is referenced in properties of other entities, we need to ensure that when the shared QNode is destroyed that we maintain referential integrity of the system by removing that QNode pointer from any properties that reference it.

This is the same as is done for QAbstractItemView when the QAbstractItemModel is destroyed.

This is responsible for a number of obscure crashes in dynamic scenes where resources are created and destroyed on the fly.