Sets a custom titlebar for this.
A typical widget used here is [class@Gtk.HeaderBar], 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 [method@Gtk.Widget.show].
the widget to use as titlebar