gnus-registry flags API

[Ted Zlatanov <tzz@lifelogs.com>]

I added an API to the gnus-registry for reading and writing flags.  It's
very simple (id is always the message ID):

;; get the list of flags (note it's a list)
(gnus-registry-fetch-extra-flags id)
;; set 'important and 'todo flags (note the parameters are not a list)
(gnus-registry-store-extra-flags id 'important 'todo)
;; remove just the 'important flag
(gnus-registry-delete-extra-flags id 'important)
;; is 'important set?
(gnus-registry-has-extra-flag id 'important)
;; remove all the flags
(gnus-registry-store-extra-flags id nil)
;; convenience function for the above
(gnus-registry-delete-all-extra-flags id)

Because of the design of the gnus-registry, these flags will persist as
a message gets moved between groups.  The flag data is *not* stored in
the newsrc file, which I think is an advantage over server-side flags
for certain situations.  Code could be written to synchronize
server-side flags with gnus-registry flags, though I think it's more
valuable as a parallel facility that backends don't use, only intended
for frontend functionality.

Caching the flags (for quick lookup of all the 'important messages, for
example) will need some extra work, as the current caching assumes that
the extras value associated with a particular message and extras key is
not a list.

The frontend functionality I would like:

- set a flag for an article (this should be a simple function a user can
call from a group or message buffer, which grabs the message ID and uses
the gnus-registry API afterwards)

- indicate the article flags somehow (fringe or status line maybe?)

- add summary line formatting support for arbitrary flags

Any suggestions or contributions (especially the three items above) are
welcome.  Please review and criticize the code if you can, I haven't
done Emacs Lisp in a bit :)

Ted