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

b/16830786

Fixes incorrect counts on TaskGroup widget on Workflow page after adding Task/Object.

* 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`.