Examples don't shut down gracefully

Some examples report "QRhi [...] going down with [...] unreleased resources that own native graphics objects. This is not nice." when closing the compositor. - This is indeed not nice. Examples are minimal-qml, custom-extension, overview-compositor, etc.

This might be solved in some compositors along the lines of:

Connections {
    target: Application
    function onAboutToQuit() {
        shellSurfaces.clear();
    }
}