WIP: define a move_email_to_account in MinimalFolder
move_email_to_account_async should let caller move an email between
folders on different accounts (see #645). It takes the same arguments as
move_email_async, plus a
I'm not shure if this is the proper way to implement this - comments requested ! It's mostly a stub ; I wonder how to make the
MoveEmailPrepare method aware of the destination account.