Merge kio_pop3 into the pop3 resource

* POPSession instanciates POP3Protocol directly
* Port POP3Protocol from TCPSlaveBase methods to using QSslSocket directly
* Replace KIO metadata with direct poking into the Settings class
* This means jobs are synchronous, which is fine in an akonadi resource (AFAIK)
* Use the Result class from kio_ftp for returning success+errcode+errstring
* Remove support for prompting for user/pass, this is supposed to be
  done in the settings dialog already. Might be convenient to
  reintroduce this somehow though, in case of a typo in the password.

The unittest passes.