void gtk_widget_set_focusable ( GtkWidget* widget, gboolean focusable )
widget can own the input focus.
Widget implementations should set
their init() function if they want to receive keyboard input.
Note that having
TRUE is only one of the
necessary conditions for being focusable. A widget must
also be sensitive and can-focus and not have an ancestor
that is marked as not can-focus in order to receive input focus.
See gtk_widget_grab_focus() for actually setting the input focus on a widget.