Name
visible? -- Return 
#t if any of WIN is currently potentially visible, else 
#f.
Synopsis
(visible? #&optional (win (get-window)))
Description
  Return #t if any of WIN is currently potentially visible, else #f.
Note that this just checks if WIN is in the current viewport
and on the current desk.  It may still return #t if WIN is completely
obscured by other windows.