#191 Configurable option for "auto mark-as-read"

[AmigaPhil]

Actually, when you open a folder, the first unread message is selected, then marked as read.
The inconvenient is that each time you enter a folder, a newer unread message is marked as read, even if you have not really read it.

I suggest to make this feature configurable.
Auto mark message as read when:
- opening message (the actual behavior)
- leaving message (that is, when selecting another message in the same folder)
- no auto-mark (user has to manually mark the message as read)

[AmigaPhil]

MAILLISTFREEZE should always disable the "auto mark-as-read" behavior, MAILLISTTHAW will restore it.
(Posting this as well as a separate request)

[Nobody/Anonymous]

There is an hidden option for that:
Hidden.DontJumpToUnreadMail=Y

[AmigaPhil]

> There is an hidden option for that:
> Hidden.DontJumpToUnreadMail=Y

Yes, it helps.
But it could be better to have this not-so-hidden. ;-)