Causes widget to become the default widget. widget must have the
GTK_CAN_DEFAULT flag set; typically you have to set this flag
yourself by calling gtk_widget_set_can_default (widget,
TRUE). The default widget is activated when
the user presses Enter in a window. Default widgets must be
activatable, that is, gtk_widget_activate() should affect them.
Causes widget to become the default widget. widget must have the GTK_CAN_DEFAULT flag set; typically you have to set this flag yourself by calling gtk_widget_set_can_default (widget, TRUE). The default widget is activated when the user presses Enter in a window. Default widgets must be activatable, that is, gtk_widget_activate() should affect them.