allout.el (allout-after-copy-or-kill-hook): New hook for extension-specific

allout.el (allout-after-copy-or-kill-hook): New hook for extension-specific
processing of killed text.
(allout-mode): Include new allout-after-copy-or-kill-hook among mentioned
hooks.
(allout-kill-line) (allout-kill-topic): Ensure that processing after kill
happens even if barf-if-buffer-read-only is raised.  Include new
allout-after-copy-or-kill-hook among that subsequent processing.
(allout-deannotate-hidden): Actually remove the annotation text properties.