[Snails] Renamed ReadStatus -> IsRead.

Better reflects the meaning of the role.
[Snails] Added MailSortModel.
[Snails] Use MailSortModel.
[Snails] Root read sorting made optional.
[Snails] Store unread children count for nodes.
[Snails] Added UnreadChildrenCount role.
[Snails] Underline items with unread children.
[Rosenthal] Added missing include.
[Snails] Convert time to local time.
[Snails] Sorting wrt to unread children.