A typical widget used here is #GtkHeaderBar, as it provides various features
expected of a titlebar while allowing the addition of child widgets to it.
If you set a custom titlebar, GTK+ will do its best to convince
the window manager not to put its own titlebar on the window.
Depending on the system, this function may not work for a window
that is already visible, so you set the titlebar before calling
gtk_widget_show().
Sets a custom titlebar for @window.
A typical widget used here is #GtkHeaderBar, as it provides various features expected of a titlebar while allowing the addition of child widgets to it.
If you set a custom titlebar, GTK+ will do its best to convince the window manager not to put its own titlebar on the window. Depending on the system, this function may not work for a window that is already visible, so you set the titlebar before calling gtk_widget_show().