void gtk_root_set_focus ( GtkRoot* self, GtkWidget* focus )
focus is not the current focus widget, and is focusable, sets
it as the focus widget for the root.
NULL, unsets the focus widget for the root.
To set the focus to a particular widget in the root, it is usually
more convenient to use
gtk_widget_grab_focus() instead of
Widget to be the new focus widget, or
NULLto unset the focus widget.
The argument can be
The data is owned by the caller of the function.