Allow inspecting scroll position of the new tree

[roblourens]

The settings tree uses these API from the old tree to implement syncing the visible elements with the TOC:

• getFirstVisibleElement, onDidScroll

This is also used to handle the case when we tab out of the virtualized tree and have to manually scroll down to get to the next element:

• getLastVisibleElement

[roblourens]

I am now using the new tree with the settings editor (much more straightforward than the search view, and the dynamic sizing works great!), but a couple things won't work quite right without this.

[roblourens]

@joaomoreno do you think you'll be able to get to this this month?

[joaomoreno]

Verification: please code review.

[roblourens]

Awesome, thanks. Is it possible to get the same behavior as the old version where it looks at how far the top element is scrolled off the page:

https://github.com/Microsoft/vscode/blob/59ba3f853d19d572d6cb3b8919d99990de338e51/src/vs/base/parts/tree/browser/treeView.ts#L668-L672

I can try it if you can tell me what's the best way to get the scroll position of an item? style.top?