Closing a window in Jalview Desktop doesn't shift focus to the previous window

Since the fix for JAL-2779 - when closing a window, the next window focused would be the most recently opened window, rather than the one most recently visited (as is normally the case).

The custom Aqua desktop manager implementation introduced to address JAL-2779 needs tweaking to properly implement the Z-stack for traversal between windows ordered according to visit, rather than creation time.