Improvements to playtree

Improvements to playtree

* Don't expand nodes by default
* Distinguish notification types from document to view
  ( update of one item, global change, append of one item)
* Handle item-append
   - Add a command/callback
   - TODO: Only rebuild the tree image if the newly appended item is
     visible
* Rename m_lastPos to m_firstPos as it represents the first position
* Beginning of a general ensureVisible function (not working yet)