add Window::visibleWindowsMostRecentFirst

That's like `visibleWindows`, but the result is ordered by 'most recently used first' (the active window is actually the first in the list).  I need that to implement Windows-like ALT-TAB, which to me is must-have. :-)

I'm very new about Objective C so my patch might not be ideal; feel free to edit/reject, but I think this function is useful.

Any case, Zephyros is great, kudos!