Moves the window into fullscreen mode. This means the window covers the entire screen and is above any panels or task bars.
If the window was already fullscreen, then this function does nothing.
On X11, asks the window manager to put window in a fullscreen state, 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 “fullscreen”; so you can’t rely on the fullscreenification actually happening. But it will happen with most standard window managers, and GDK makes a best effort to get it to happen.