Add setting for whether to display scrollbars

[gskinner]

We should probably wrap our scroll views (ScrollGroup & Repeat Grid) in a Scrollbar by default.

I wonder if we should also add a setting for toggling it on or off? Are there common use cases for not having a scrollbar? Feedback welcome.

[gskinner]

8bcf84a adds scrollbars. Leaving open to consider whether to add a setting.