Prevent sending multiple WINDOW_CLOSED events for already disposed windows

Reported at: http://mail.openjdk.java.net/pipermail/awt-dev/2013-May/004749.html

> I reported this issue one year ago but seems that it has been removed
> from the public bug database and is still not fixed so I try now to
> speak directly with you to see if this really is a bug or not.
>
> The problem that I see is that a child window already disposed is
> re-disposed when the owner frame is disposed, multiplying the
> WINDOW_CLOSED events.
>
> I guess that it could be fixed by testing if the child window is
> displayable before disposing it in the Window dispose action. (May be
> better in the dispose method itself).