(Vwindow_list): New variable.

(Vwindow_list): New variable.
(make_window, delete_window): Set Vwindow_list to nil.
(check_window_containing): New function.
(window_from_coordinates): Rewritten.
(add_window_to_list, window_list, candidate_window_p)
(decode_next_window_args, next_window): New functions.
(Fnext_window, Fprevious_window): Rewritten in terms of
next_window.
(Fwindow_list): New function.
(Fother_window): Cleaned up.
(foreach_window): Add a longer "variable argument list".  Let
callback function return 0 to indicate that cycling over windows
should stop.
(foreach_window_1): Likewise.
(freeze_window_start): Return int.
(init_window): New function.
(syms_of_window): Staticpro Vwindow_list and defsubr Swindow_list.