Add KWindowStateSaver

This is basically the C++ counter-part to !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.