mail_daemon: cleanup, 30s auto check startup delay.

* Refactored new message retrieval a bit, so that the notification strings
  aren't duplicated.
* The daemon now waits 30 seconds before doing the first mail check.
IMAP: make the body fetch limit setting available.

* This is set by the ProtocolConfigView -- there should really be
  some constant for this.
IMAP: if connecting fails, try again a few times.

* Also documented SyncCommand.
IMAP: work in progress of downloading the mail body.

* Most things are in place now, we just try to download the body to the
  wrong file, as the final location is currently unknown.
* Added local only kPartialMessage flag for mails, but it's not being
  used yet.