(enum window_part): New enumeration.

(enum window_part): New enumeration.
(Qleft_fringe, Qright_fringe): Replace Qleft_bitmap_area and
Qright_bitmap_area.
(coordinates_in_window): Return an enumerator from enum
window_part.  Fix handling of right scroll bar.
(Fcoordinates_in_window_p): Doc fix.  Return `left-fringe' and
`right-fringe' instead of `left-bitmap-area' and
`right-bitmap-area'.  Use enumerators from enum window_part
instead of hard-coded integers.
(check_window_containing, window_from_coordinates): Use
enumerators from enum window_part instead of hard-coded integers.
(syms_of_window): Replace Qleft_bitmap_area and
Qright_bitmap_area.with Qleft_fringe and Qright_fringe.