Caching of msg-id to org-id correlations

lisp/gnorb-utils.el: New variable `gnorb-msg-id-to-heading-table' to
		     hold the hash-table cache. Calling
		     `gnorb-trigger-todo-action' adds an entry to that
		     table.

lisp/gnorb-org.el: Now function `gnorb-org-find-visit-candidates' just
		   queries the cache, or fills it. Filling done by new
		   function `gnorb-org-populate-id-hash', with the help
		   of `gnorb-org-add-id-hash-entry.

lisp/gnorb-gnus.el: Remind users that finding todo candidates only works
		    with `org-id-track-globally' set to t.
Collect attachments on incoming trigger action

lisp/gnorb-gnus.el: `gnorb-gnus-incoming-do-todo' now collects
		    attachments from the incoming message.

lisp/gnorb-util.el: And then `gnorb-trigger-todo-action' prompts for
		    attachment.
Hinting for potentially-relevant incoming messages

Now that we have a quick-lookup cache for finding correlations between
messages and todo items, we can do more interesting stuff. Opening
messages can automatically flash a hint if the message is relevant to a
todo. A special mark can also be displayed next to relevant messages in
the *Summary* buffer.

lisp/gnorb-gnus.el: New options `gnorb-gnus-hint-relevant-article',
		    `gnorb-gnus-summary-mark-format-letter' and
		    `gnorb-gnus-summary-mark' control the above
		    behavior. New function
		    `gnorb-gnus-hint-relevant-message' is added to the
		    `gnus-article-prepare-hook', and a
		    gnus-user-format-function is constructed for the
		    appropriate format letter.
Slight improvement to point placement in new messages

lisp/gnorb-org.el: `gnorb-org-setup-message' is a little smarter about
		   leaving point where you want it.
Improvements to incoming message handling

lisp/gnorb-gnus.el: Function `gnorb-gnus-incoming-do-todo' now checks
		    all the referenced msg-ids for relevance.
Improvements to outgoing todo handling

lisp/gnorb-gnus.el: Function `gnorb-gnus-outgoing-do-todo' has been
		    largely re-written to do a better job of locating
		    all possible references, and handling them. Function
		    `gnrob-gnus-check-outgoing-headers' now stores ids
		    from the References header, allowing us to find
		    relevant todos even after a reply to an incoming
		    message is already sent.