Messages int he Sent folder could have missing From header when sent by
the evolution-ews. The From/Sender is still available, only not present
in the MIME content, thus fill it if necessary.

Some folders can contain not only mail messages, but also pure
calendar/contacts/... items, which the mail (Camel) folders do
not operate with well, but more importantly which do not belong
there, thus skip those items in mail folders.

This is related to the evolution-list mailing list thread:
https://mail.gnome.org/archives/evolution-list/2015-September/msg00013.html

It could happen that one connection had been reused by multiple
backend authentication attempts, one effectively unsetting password
for the other, causing password prompt for the EWS account.

Also listen for ESource changes only after the backend is populated,
which avoids unnecessary server pings on start.

Without it a crash could happen when the store's sync state was invalid
for some reason and a fresh folder hierarchy discover was run.

It can happen that the notification subscribe and unsubscribe work
properly, but the actual GetStreamingEvents request fails with
a client or a server error. it doesn't make sense to keep looping
in Subscribe/Unsubscribe requests in such cases.

Aka skip virtual, public and foreign folders from the list of
folders to listen on, which is not allowed by the server anyway.