(prop_location_identifier): Was named prop_location_tick.

(prop_location_identifier): Was named prop_location_tick.
(property_change_reply_identifier): Renamed from ...-tick.
(struct prop_location): Field `identifier' renamed from `tick'.
Various functions changed accordingly.
(x_handle_selection_request): Delete local struct var `reply'.

(wait_for_property_change_unwind): New function.
(wait_for_property_change): Add unwind protect.

(wait_for_property_change): Eventually time out with error.
(x_reply_selection_request): XFlushQueue and UNBLOCK_INPUT
before calling wait_for_property_change.
(x_get_window_property): Simplify input blocking/unblocking.
Don't delete the property here.
(receive_incremental_selection): Delete property here.
Call XSelectInput to enable and disable PropertyChangeMask.
(x_get_window_property_as_lisp_data): And here.