Provide user with list of known logins on login screen

[okdistribute]

from @hpk42:

Each DB is tied to exactly one account and vice versa.
dc_context is kind of a synonym for "dc_account".
The way to have multiple accounts is to have multiple DBs.
in the python bindings i happily have 2 active accounts
with their respective smtp/imap threads.

If you have a directory with different DBs and want
to show a selector of the different accounts,
say db1, db2, db3 in a particular application dir,
you can make a for-loop over the dbs, opening each, checking if
is_configured() is true, getting the "addr" config, and closing it.

There is no need to have a db-filename that is derived from "addr".
And this means that in "single account" mode (the one we are aiming to
get working) you can open "db1", start threads and don't ever stop the
threads or close/unref it unless you are really switching accounts.
You can set_config(addr), set_config(mail_pw) and dc_configure()
until success.