Ensure that dynamic property storing QObject ptr notifies on delete

Ensure that dynamic property storing QObject ptr notifies on delete

Previously, when a QObject ptr was stored in a dynamic variant
property, the value of the property could change (to a zero ptr)
if the QObject was deleted without a notify signal being emitted
by the QDeclarativeVMEMetaObject which stores the property.

This commit ensures that such a notify signal is emitted correctly.

Task-number: QTBUG-23451
Change-Id: I5689abd984b177737f8d5f18950838b73ebde328
Reviewed-by: Martin Jones <martin.jones@nokia.com>