Make SketchBottomBar scroll controller available

[RoBoT095]

I wanted to add a Scrollbar with always visible thumb bar to the SketchBottomBar, as I myself didn't realize it was scrollable, but It gives out error:

The Scrollbar's ScrollController has no ScrollPosition attached.

I believe it just needs to make SketchBottomBar's SingleChildScrollView have the same controller.

[JunYeong0314]

Great catch! I honestly hadn't thought of that. A visible scrollbar is a perfect solution. Thanks for the great suggestion! I'll try to get this into a future update, but a PR from you would be awesome too if you're up for it.