This adds `Window::set_window_level` to control the preferred
z level of the window.
Co-authored-by: Markus Siglreithmaier <m.siglreith@gmail.com>
Co-authored-by: Kirill Chibisov <contact@kchibisov.com>
Co-authored-by: Mads Marquart <mads@marquart.dk>