Move Maildir syncback support into main sup

Tracking issue for maildir-syncback.

It would be great to get Maildir syncback support into one of the next releases. I gather it has been tested pretty extensively over the years now and should be more mature than most new things.

I would like to release the first 0.14 without maildir syncback, but then merge it in for one of the immediate releases afterwards. There are a lot of big changes in 0.14 which would be nice to keep in a separate release so that users can downgrade if maildir-syncback doesn't work out.

(@ericweikl poke)