(struct property_change): New field `arrived'.

(struct property_change): New field `arrived'.
(expect_property_change): Return struct property_change *.
Clear `arrived'.
(unexpect_property_change): Take one as argument.
(wait_for_property_change): Take one as argument.
If `arrived' is set, don't wait.
(wait_for_property_change_unwind): Corresponding changes.
(receive_incremental_selection): Corresponding changes.
(x_reply_selection_request): Corresponding changes.
(property_deleted_p): Function deleted.

(x_handle_selection_clear): Redisplay.