* window.c (enum window_part): Move to dispextern.h.

* window.c (enum window_part): Move to dispextern.h.
(coordinates_in_window): Use enum window_part member names
instead of numbers to describe return value.
(struct check_window_data): Change part member to window_part.
(check_window_containing): Return window_part unaltered.
(window_from_coordinates): Change part arg from int to enum
window_part.  Allow part arg to be null.  All users changed.