emacs: Helpers needed for the user to be able to choose the "From" address when compo...

emacs: Helpers needed for the user to be able to choose the "From" address when composing a new message

This adds functions and variables needed for this feature to be implemented.
Once it's done, the user will be able to use a prefix argument (e.g. pressing
C-u m instead of m) and be able to select a From address.

By default the list of names/addresses to be used during completion will be
automatically generated by the settings in the notmuch configuration file. The
user can customize the notmuch-identities variable to provide an alternate list.

This is based on a previous patch by Carl Worth
(id:"87wrhfvk6a.fsf@yoom.home.cworth.org" and follow-ups).