On removing sequence_number from window structure

[martin rudalics]

I'm afraid this change

  * window.h (struct window): ...
  Remove sequence_number member. ...
  * window.c (sequence_number): Remove.
  * print.c (print_object): Follow the printed representation of
  frames and print window pointer to distinguish between windows.

will make debugging window code rather unpleasant.  It's much simpler to
think of windows in terms of sequence numbers than in terms of pointers.

martin