ScrollablePositionedList: offset and listen offset

[CaiWanFeng]

Need offset like ScrollController's offset. Can you expose ScrollController? This is very useful and I think many people desire.

[simplenotezy]

That would be very nice, something we need as well

[VusDanylo]

+1

[quangtuyen1993]

+1

[tarobins]

Please see if changes in #474 and #472 suit your needs.

[lenzpaul]

Please see if changes in #474 and #472 suit your needs.

While the ScrollOffsetListener partially address the issue, it does not get notified when the ItemScrollController moves via a call to ScrollOffsetListener.jumpTo

@tarobins