libbuxton: Update buxton_get_value to return an int
libbuxton: Update buxton_register_notification to return an int
libbuxton: Update buxton_unregister_notification to return an int
libbuxton: Update buxton_set_value to return an int
libbuxton: Update buxton_set_label to return an int
libbuxton: Update buxton_create_group to return an int
libbuxton: Update buxton_remove_group to return an int
libbuxton: Update buxton_client_list_keys to return an int
libbuxton: Update buxton_unset_value to return an int
demo: Add buxton_unregister_notification example code