Set if window must be kept below other windows. If the window was already below, then this function does nothing.
On X11, asks the window manager to keep window below, if the window manager supports this operation. Not all window managers support this, and some deliberately ignore it or don’t have a concept of “keep below”; so you can’t rely on the window being kept below. But it will happen with most standard window managers, and GDK makes a best effort to get it to happen.