It's bound to happen sooner or later. You're working away on your latest masterpiece, then BAM! A window in paint.net disappears!     Lawks! What to do?? Quick - do this.....Use the function keys F5 to F8 to toggle the missing window back into view.  So which function key does what?     To restore the Tools Window     To restore the History Window     To restore the Layers Window     To restore th