Make core_ext entry for TreeItems
Use more API from dsl for tree_item
Add based core_ext for tree_views
Whoops errant puts
Simple example of using a tree view
Some more missing classes to be imported
A fairly complex example of tree view with drag and drop support
New default converter argument (pushing API to limits). drag_event converter
Fix bug when dragging on part of tree view which has no tree_items.  Also make tree view editable