Once #3820 is done, which fixes

• Create new ScrollBar based on a new Scroll and remove ScrollBarView/ScrollView #2489

we will want to upgrade TreeView to support ScrollBar. There are two approaches:

1. Upgrade TreeView to utilize the virtual content scrolling now built into View via Viewport, ViewportSettings, and SetContentSize().
2. Leave the existing scroll logic in place, and program View.HorizontalScrollBar/VerticalScrollBar manually.

The former is a far better solution and should not be too hard to accomplish.