Just a side note: I am amazed how people are obsessed to have no "unread" messages even in the Trash.
I understand the workflow when people might want to have no unread messages in the Inbox, - where "unread" is an indication for them to read or deal (save to a folder, delete) with those messages.
But Trash? Yes, that folder can be highlighted that it has unread messages, so what?! It is not that you would normally look in the Trash for messages that you need to read. I guess it is just some type of OCD.
So, it's nice of Kostya that he made it configurable for those users.