- make autoplayDelay can change in onIndexChange

[majdkz97]

if we have multiple images and autoplay is on.
and we need to make image durations different so i make this small edit on swiper.dart.

to use this feature:

onIndexChanged: (index) {
                    setState(() {
                      autoPlayDelay = images[index].durationYouWant;
                    });
                  },