The attached patch adds syncing the INTERNALDATE of IMAP folders with
the mtime of messages in maildir folders.
I want this to happen, because I'm running a dovecot over the maildirs
synced by offlineimap, and that uses the mtime as the INTERNALDATE.
When using mutt to view messages I generally sort based on the received
date, which for IMAP folders is the INTERNALDATE.
Since this is the first real coding I've done in Python the patch may
need to be cleaned up some, but it's working pretty well for me. I've
added new messages to each side, and the received date has been
preserved going both ways.