add "signals" for start and end of screenshot capturing. Like the other

add "signals" for start and end of screenshot capturing. Like the other
signals, this is meant for attaching listeners. The ufo will use that to
hide/reveal the status line in screenshots. The following signal properties
are now available:

  /sim/signals/exit       ... set to 1 right before quitting
  /sim/signals/reinit     ... set to 1 on re-init  (Shift-Esc)
  /sim/signals/screenshot ... set to 1 before and to 0 after screenshot
  /sim/signals/click      ... set to 1 after mouse clicks at terrain, signalling
                              that the geo coords in /sim/input/click/ were updated