Bring GMail labels section on the manual up to date

 - there is no need to set SQlite backend anymore;
 - explain a bit more that some headers are recognized
   by email clients.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Copymessageto should not be private since we override it

private methods prevent them from being overriden on derived classes. In
GmailFolder we need to override copymessageto, so it can't be private.

Before this commit, copymessageto was made private in Base but not in
GmailFolder. The end result was that labels were not set when copying
the message content, and always needed to be set on the label copying
pass.

Pointyhat-to: Eygene Ryabinkin
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Extend handling of cert_fingerprint

Add ability to specify multiple fingerprints.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>