Tree.Sortable: Add deep sorting

This change adds two features to `Tree.Sortable`:

The new `Tree.Sortable#sort()` method sorts every node in the tree.

The `Tree.Sortable#sortNode()` method now has a `deep` option. If set to `true`, every node in the specified node's hierarchy will be sorted instead of just the children of the specified node.

Inspired by #867.