[enhancement, major] Accordion-style tree GUI widget

[amyadzuki]

1. Let us mark a tree GUI widget as "accordion style" where only one TreeNode can be opened at a time. Opening a TreeNode when another one is already opened in the same Tree automatically closes the previous one.

2. It would be especially nice if we could .Subscribe to the opening of a TreeNode and have a callback that we specify called when that TreeNode is opened. I would use this in my character designer to zoom in on the face when you open a TreeNode related to customizing a part of the face (such as "Eyes") and zoom out to show the full character when you open a TreeNode related to the body (such as, well, "Body").

[danaugrs]

@amyadzuki I agree this would be a nice improvement.