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

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).