Tracking: Auto-scroll the tree while dragging

[TrevorBurnham]

Summary

Dragging a node toward the top/bottom edge of the viewport should auto-scroll the list so users can drop onto off-screen rows. Today this is unreliable (tiny hot-zone) or broken in some browsers.

Rolls up

• Allow user to scroll while dragging a node #323 Allow user to scroll while dragging a node
• Better scrolling while user is dragging  #226 Better scrolling while user is dragging
• Drag and Scroll #106 Drag and Scroll (Safari does not scroll the container)

Related but tracked separately: #191 (touch DnD).