Sets the focus widget.
focus is not the current focus widget, and is focusable, sets it as the focus widget for the window. If
null, unsets the focus widget for this window. To set the focus to a particular widget in the toplevel, it is
usually more convenient to use [method@Gtk.Widget.grab_focus] instead of this function.
widget to be the new focus widget, or null to unset any focus widget for the toplevel window.