Private messages: Add new Read column to Outbox tab

As a user, I want to know when recipient read my sent message.
Please, add column 'Read' to Outbox tab.

Let's see an example:

• Alice creates a message and send it to Bob.
  Message is shown in the Outbox, sent time is set, read column is empty
• Bob login to forum, but not read a message: no changes in Alice's Outbox.
• Bob reads a message. Read time is set in Alice's Outbox.
  Sent time is still shown in Inbox (both Alice and Bob)

Also please rename Date column to Sent.