Improve custom node management

Managing nodes that are derived from TreeListNode is a bit tedious atm.
* the CreateCustomNode event handler doesn't have additional information about the type of node that should be created
* extra properties of the nodes can only be set after the CreateCustomNode event handler has finished. In between, FocusedNodeChanged may be raised with the node, which then might be missing some important properties.
Proposed Solution:
Add another argument to XtraTreeList::AppendNode(), say, "object userData".
This reference would then be added to CreateCustomNodeEventArgs.
This means that users can pass arbitrary arguments to the CreateCustomNode event handler, which would solve the issues outlined above.
An even nicer solution would be to offer another AppendNode() method that just takes an instance of a TreeListNode. Not sure how much trouble that could cause internally though.
Working around this issue is pretty annoying, so it would be cool if you could do something about it.