GtkWindow::set_focus
Sets the focus widget for the window to focus. If focus is null, the focus widget for the window will be unset.
If focus is not a focusable widget (ex.
GtkLabel), a Gtk-CRITICAL error
will be thrown.
The focus widget is the widget that will be given keyboard focus when the window receives keyboard focus.
See also: activate_focus() , get_focus() , grab_focus()