pyqt broke sip4 compatibility in 5.15.6, and there is no more maintainer
of the KF Python bindings to fix things.

Future support might need different code, so no advantage in keeping the
old code around.

This is basically the C++ counter-part to
https://invent.kde.org/frameworks/kconfig/-/merge_requests/94
and allows to easily retrofit window size persistence on existing windows/
dialogs, replacing e.g. code like
https://invent.kde.org/pim/pimcommon/-/blob/master/src/pimcommon/widgets/kpimprintpreviewdialog.cpp.

This is a bit more complicated than one might expect, as KWindowConfig
works with QWindows, but that's something freshly created QWidget windows/
dialogs don't have yet. Additionally, we are in a library here that doesn't
depend on Qt::Widgets. To overcome this we move the widget-dependent code
(basically just a call to QWidget::windowHandle()) to inline template code
(and thus into the consumer), use std::function's type erasure to pass it
into the library code, and an event filter on the widget to wait for the
QWindow to become available.

Macros Q_NAMESPACE_EXPORT should be used without semicolon
https://doc.qt.io/qt-5/qobject.html#Q_NAMESPACE_EXPORT

This generates warning: extra ‘;’ [-Wpedantic]

This fixes the build with Qt6 on Android.

This has should be done explicitely, as the docs explain.

This causes issues for the KScreenLocker KCM port.