Change `SortableTreeView` to `sortable` option on `TreeView`

* Currently, `updateCount` propagation suppression works by checking for
  the controller class name on the element, which breaks if the subtree
  has a different class name.
* `SortableTreeView` was a simple enough change to move to be just an
  option on `TreeView`.
Merge pull request #1621 from reciprocity/feature/remove-sortable-tree-view

Change `SortableTreeView` to `sortable` option on `TreeView`