wish: notmuch-emacs: tag unsent email with same key bindings as in notmuch show

[Gregor Zattler]

Hi notmuch developers, it's super fast to use "k" + key binding
to tag mails in notmuch-emacs show, it's great that one may tag
unsent emails via the Fcc: header, but even there is C-c C-f C-w
in message mode it's error prone and especially slow to edit tags
without UI support.

It would be great if in notmuch-emacs the same tagging bindings
worked for unsent mails (obviously not with "k" but another key
combo as prefix key ["C-c k" is not bound in message mode at
least in my configuration]) as in notmuch show.

Thanks for your attention, Gregor