Add an onClose event to the UIWindow

At the moment I'm creating a UIButton in order to execute some logic before closing the window. It would be really cool to have an onClose event for the UIWindow that will execute some logic without having to add a separate button to the window.

Start by locating UIWindow and its existing close path, then inspect how UIButton actions are wired. Define done as allowing callers to attach an onClose handler that runs when the window closes, without requiring a separate button; add coverage if the relevant test area becomes clear.

Written by the indexing model from the issue text.