Add support for custom getters and setters in QProperty

If I understand correctly, currently QProperty allows only trivial getters and setters.

Custom getters can be achieved by reimplementing QBindableInterface's getter() and makeBinding() methods. However this approach is quite verbose (see example here: https://codereview.qt-project.org/c/qt/qtbase/+/327038/6/src/corelib/itemmodels/qabstractproxymodel_p.h).

The situation with setters is even worse. We do have a Q_OBJECT_COMPAT_PROPERTY macro, that allows a custom setter, but if we just want to use QProperty, there is no way for now. Trying to achieve that, I had to create my own class, which was a combination of QProperty and QObjectCompatProperty: https://codereview.qt-project.org/c/qt/qtbase/+/327265/3/src/corelib/kernel/qcoreapplication_p.h