[scrollable_positioned_list] Expose _ScrollablePositionedListState

[TheCarpetMerchant]

Please expose _ScrollablePositionedListState So as to allow us access to primary.scrollController.
I need to scroll a precise amount of pixels, and this isn't possible with the current API.

[TheCarpetMerchant]

Nevermind I didn't think to check the PRs before, and #305 will actually solve my problem, since all I need is access to the ScrollController. I'll do the same thing while waiting for this to be merged.