Avoid qt traceback when clipboard has no python data object.
qt:tree_editor: prevent errors on drag-move of items.

QTreeWidgetItem does not have equality operator implemented,
so we check against id().
Verification: traitsui/examples/demo/Standard_Editors/TreeEditor_demo.py
 - Drag any employee from 'Scientific' to 'Business' dept.
Qt:tree_editor: prevent NoneType error on drag over top level item

Verification: traitsui/examples/demo/Standard_Editors/TreeEditor_demo.py
 - Drag any file from file-browser (finder/explorer/nautilus)
   over the top level node labelled 'Enthought'
Merge pull request #92 from pankajp/pymimedata-coerce-no-instance

Avoid qt traceback on drag/drop/clipboard operations.