QProperty: Notfication missing if binding no longer has any dependencies

If we re-evaluate a property's binding due to a another property's change, and the binding has no further dependencies on any QProperty, then we won't send out a change notification, even when the property now has a different value.

This can happen when the binding captured some mutable state that is not QProperty based (else there would be no way to go from >0 to 0 dependencies).

This is an unintended consequence of the split between binding evaluation and notifications