[Review Request] Provide API to expose list of showing Windows

[Jonathan Giles]

Hi Kevin, all,

Please review the most recent patch attached to the following JBS issue:

https://bugs.openjdk.java.net/browse/JDK-8144628

This API removes the Window.impl_getWindows() method that returned an 
Iterator<Window>, and replaces it with Window.getWindows(), which 
returns an unmodifiable ObservableList<Window>.

Thanks,
-- Jonathan