Store link in message mode

[Ulf Stegemann <ulf-news@zeitform.de>]

After it's now possible for me to store an org link upon sending a
message[1] I thought it might be even better to approach this problem in
a more general way.  The idea is to use `org-store-link' in message mode
to create a link for the (yet to come) copy of the message.

Currently `org-store-link' seems to be rather pointless in message mode
as it creates a link to a draft (temporary) message file.  Extending
`org-gnus-store-link' to let it create a gnus link to the copy of the
message when in message mode seems to be more reasonable.

However, there are some, er, `issues' with this.

1) This will only work with Gnus.  Message mode is in principle
   independent of Gnus but non-Gnus users will not benefit from
   `org-store-link' in message mode.

2) Storing a link in message mode effectively creates a link to
   something (the mail copy) that's (still) inexistent.  This means that
   the link remains invalid until the message is actually sent (or more
   precisely: the copy is created upon sending).

   Furthermore, the link will be invalid if

   - the message is not sent;
   - the Gcc header is modified to point to a different group;
   - the message id is changed.

   And of course, the link description will be incorrect if Subject,
   From and/or To headers are changed between calling `org-store-link'
   and creating the message's copy.

If you think --despite of those issues-- it's worth adding the creation
of gnus links while in message mode I could provide a patch.  I'd
probably add the functionality to `org-gnus-store-link' but I'm not an
org code expert and a different location may be more appropriate.  What
do you think?

Ulf

References:
[1]  see http://thread.gmane.org/gmane.emacs.orgmode/31280