[Users] XOAUTH2 and claws-mail

[David Fletcher]

The relevant information about the protocol exchanges to be performed are
here:
https://developers.google.com/gmail/imap/xoauth2-protocol
https://developers.google.com/identity/protocols/oauth2#2.-obtain-an-access-token-from-the-google-authorization-server.
https://developers.google.com/gmail/imap/xoauth2-libraries

I don't fully understand the refresh token (vs access token), but
that's probably a secondary issue to look at.

It appears that eventual use requires some verification of the software
by google, but for development I can treat it as a personal project for
accessing just my own account. This tutorial steps through things: 
https://afterlogic.com/mailbee-net/docs/#OAuth2.html

I have version Claws Mail 3.17.5 compiling from source without errors or
missing libraries so I can start to investigate now. If it's productive
I'll send the patches! I've been running Sylpheed and later Claws Mail
since early 2000s and would prefer to keep using it.

David.