If focus is not the current focus widget, and is focusable, sets
it as the focus widget for the window. If focus is 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
gtk_widget_grab_focus() instead of this function.
If focus is not the current focus widget, and is focusable, sets it as the focus widget for the window. If focus is 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 gtk_widget_grab_focus() instead of this function.