mt_widget_set_data — sets user data to widget
void mt_widget_set_data(MT_WIDGET *widget, void *data, mt_destroy_func data_dispose);
Sets user data to widget. You can set only one user data for each widget. You can optionally pass the function which will properly release user data from memory. It's called when the widget is destroyed. It's possible it will never be called when the application exits.
To remove user data, simply pass NULL as data.