void gtk_window_set_auto_startup_notification ( gboolean setting )
By default, after showing the first
GtkWindow, GTK+ calls
gdk_notify_startup_complete(). 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, for example.
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.