Sets whether the window should request startup notification.
By default, after showing the first
GtkWindow, GTK calls
gdk_toplevel_set_startup_id(). Call this function
to disable the automatic startup notification. You might do this
if your first window is a splash screen, and you want to delay
notification until after your real main window has been shown,
In that example, you would disable startup notification temporarily, show your splash screen, then re-enable it so that showing the main window would automatically result in notification.