GtkWindow::maximize
Tells the window manager to maximize the window. If it's maximized,
it takes all the available space which is not occupied by extra windows
like e.g. the task bar.
It may be that there are some settings preventing this, so don't depend
on that it is really fullscreen after calling this function.
Normally it is, but don't write code that crashes if not.
See: unmaximize()