QPropertyChangeHandler is a templated class and it's argument is
a functor. That makes it inherently cumbersome to use the class
in any context where the change handler needs to be stored.
Introduce a QPropertyNotifier class that stores the functor
in a std::function<void()>, and add a QProperty::addNotifier()
method that can be used instead of onValueChanged().
Also make QPropertyNotifier default constructible.
This significantly simplifies the code that needs to be written
and makes it possible to store notifications as class members
without major hassle.
Fixes: QTBUG-92980
Change-Id: Id5b7baec093b9ac0467946cded943d92ad21030b
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
6c1a9f2b4d