Move scroll code into `onSelect`.
Move scroll code into `onSelect`.

This way it's fired when a tree is selected outside of the
`.select(node)` API call.

Most of this change is moving the code into a function; The real bug fix
is making sure it's called in `._onSelect` rather than `.select`

Fixes #301

SpiderStrategies/kalpa-tree,SpiderStrategies/kalpa-tree