TreeEditor will not expand TreeNode if "auto_open" is not set to True

a TreeNode will not be expanded if TreeNode's auto_open attr is not set to True.
this renders the TreeEditor pretty unusable, since it becomes impossible to navigate, when working with a large number of nodes.

The offending code is likely part of the traitsui.qt4.tree_editor.SimpleEditor's "_insert_node" method:

```
# Automatically expand the new node (if requested):
if has_children:
if node.can_auto_open( object ):
cnid.setExpanded(True)
else:
# Qt only draws the control that expands the tree if there is a
# child. As the tree is being populated lazily we create a
# dummy that will be removed when the node is expanded for the
# first time.
cnid._dummy = QtGui.QTreeWidgetItem(cnid)
```

Likely the ._dummy fix is the offending statement here.

Possible this bug is platform dependent, this was tested on OSX