widget handlers react on events; used by widget implementation 


struct mbtk_handler_s {
  const char *name;                                        /* cookie; a handler implementation will always use the same pointer for name so it can be checked with ==; also useful for debugging */
  mbtk_event_handled_t (*event)(mbtk_widget_t *widget, mbtk_event_t *ev);   /* called when an event is received for a widget (or child of that widget) that has the handler associated; if event is not handled it is propagated up in the widget hierarchy */
  void (*widget_remove)(mbtk_widget_t *widget);            /* called when one of the widgets of the handler is destroyed or for other reason the widget should be removed from the handler */
  void (*handler_destroy)(mbtk_handler_t *hand);           /* called when the parent window of the handler is destroyed */
  gdl_elem_t link;                                         /* linked list of handlers in a window (for destroying handlers when window is destroyed); only when dynamically allocated */
  /* over-allocated with handler specific fields */
};