[MlMt] Replacement for MmHeadersView

[Jonathan Van Eenwyk]

I've been using Chauncey's correspondence layout 
(https://github.com/chauncey-garrett/mailmate), but it doesn't seem to 
work with the MailMateBigSur version 
(https://updates.mailmate-app.com/archives/MailMateBigSur.tbz) because 
"MmHeadersView" is no longer available. What's the replacement for this 
view? With a little guidance, I'm happy to update the layout as needed.

Thanks!

Jonathan