(it_props): Add an entry for composition.

(it_props): Add an entry for composition.
(face_before_or_after_it_pos): For composition, check face of a
character after the composition.
(handle_composition_prop): New function.
(get_next_display_element): Adjusted for the change of
CHAR_STRING.
(set_iterator_to_next): Handle the case that it->method ==
next_element_from_composition.
(next_element_from_composition): New function.
(message_dolog): Adjusted for the change of CHAR_STRING.
(set_message_1): Likewise.
(check_point_in_composition): New function.
(reconsider_clip_changes): If point moved into or out of
composition, set b->clip_changed to 1 to force updating of the
screen.
(disp_char_vector): Delete codes for a composite character.
(decode_mode_spec_coding): Adjusted for the change of CHAR_STRING.