The -[NSWindow isVisible] method returns FALSE when the process is hidden,
but that's not what we need to know in some cases.

This fixes full-screen games which minimize their window when they lose
focus.  Command-Tabbing away hides the process.  Because the window was not
visible, the code didn't actually minimize it.  When switching back to the
process, no event was sent to the Wine back-end telling it the window had
been restored, so it never resumed drawing to it.

While the process is hidden, it can't get a list of its windows which would
be visible were it not hidden.