Update message in thread index

This patch let's updated changes propagate to the thead-index-mode. Some changes were made to ensure a add_unhide event re-loaded the message. Including deleting and reloading the thread.

Also, because there seems to be some issues with the locations of a message all messages returned with :add are sent to the UpdateManager.

The imap old messages stuff now yields updated messages so that they are sent to UpdateManager.