Fix race conditions in TreeView

[roelvandergoot]

When you:
1.Represent a directory tree as a TreeView,
2. Use a state machine go-routine per leaf TreeNode, and
3. Append the current state name of the state machine to the TreeNode's using TreeNode.SetText(),
a race condition exists between TreeNode.SetText() and TreeView.Draw()

You can't solve the race condition by embedding rivo.TreeView and rivo.TreeNodes in your own TreeView and TreeNodes. Hence this pull request.

[roelvandergoot]

Let me know if you want me to rename mu or place it somewhere else in the TreeNode struct or add mus to other widgets. Then again you can also feel free to make your own alternate (better according to you) merge request. :)

BTW. I love this repository! Thank you very much for providing it.