TreeView - add onCollapse function

[edewit]

Is this a new component or an extension of an existing one?
What is the existing component, if any?
TreeView

Describe the feature
A clear and concise description of the new feature. What is the expected behavior?
Add the ability to add an onCollapse listener to the TreeView. I would like to load the children of the node dynamically this listener would be needed to inform me of when the user opened the tree node.