make it so you can view "threads with unread". (#52927)
threaded and unread are no longer mutually exclusive.
later, I'll add "unread threaded" which is no the
same thing as "thread with unread".
fix the nsMsgMessageDataSource() to ask the view, thread or msg database for existance of messages, instead of getting an enumerator for all messages, and seeing if it is non empty. more performance work on the way.
move enums from MailNewsTypes.h to MailNewsTypes2.idl, so that I can use the same ones from JS and C++.
sr=bienvenu