When using QQuickDeferredPointer for deferred properties in Qt Quick
Controls, it is necessary to also implement the itemDestroyed virtual
method because if the item pointed gets deleted the
QQuickDeferredPointer reference will be a dangling pointer when cleanup
occurs in the destructor. This patch is following the pattern laid out
by QQuickControlsPrivate for handling its QQuickDeferredPointer members.

This bug is exposed when aborting a component incubation for types that
did not do this cleanup in the form of a segfault in the destructor of
the offending class.

Fixes: QTBUG-116828
Pick-to: 6.6 6.5
Change-Id: I6a486de74ee38ccdcaff6a31195ee389f51fc93c
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>