TreeNode is not "exported"

[Midnighter]

To be added by @willmcgugan

I'm adapting the DirectoryTree class for my own purposes and I noticed that I needed the TreeNode class to provide the right type hints. This class is currently considered "private" rather than public to textual.widgets. I don't see why it shouldn't be made public for purposes of subclassing TreeControl.

Thank you for considering it.