Does not update when `value` is changed from outside

Given code like this:

```
TouchSpin(
  min: 0,
  max: 99999,
  value: outsideValue,
)
```

Then the `TouchSpin` widget does not update if `outsideValue` is changed on the next rebuild. `outsideValue` may come from state, bloc etc.

Tested w/  `flutter_touch_spin: ^2.0.0-nullsafety.1`