Mark Message as Read hook event

[JDGrimes]

PMs sent by the messages component can be marked as read and unread. This event would fire when they were marked as read, and reverse when they were marked as unread.

The odd part though is that these apply to entire threads at once, not individual messages, and so the actions fired by BP_Messages_Thread::mark_as_read() and BP_Messages_Thread::mark_as_read() only pass the thread ID.