raygun: Implement unread status

[dariusc93]

In the current implementation, there is no way to tell if a message has been read or seen by a user. Instead, this would likely rely on more additional logic in any frontend implementation. On top of that, no event is emitted to the conversation when the message is read to change the status of the message itself.

Here, I believe that it would be better to add functionality to allow RayGun to inform the user of any unread messages, either by adding a option to MessageOption so RayGun::get_messages would list unread messages. We could also add a option to only count unread messages with RayGun::get_message_count to determine any messages that havent been read by the local node, while:

1. Adding an argument to to RayGun::get_messages and RayGun::get_message to mark any messages fetched as read; or
2. Add a new function called RayGun::read_message, which would mark a set of messages as read.

An event can be emitted to both the local node as well as the participant in the conversation about the message being read for direct messages. For group messages, this could possibly be ignored, although we could still emit an event about the message being read when it comes in, but may not be desirable in such a setting.

In addition, we can add a new status to MessageStatus, possibly called MessageStatus::Read to indicate if any successfully sent or receive message have been read, and possibly provide a list of those have read it.

Note:

• If the recipient read the message, it cannot be unread.
• If a list of peers is supplied to MessageStatus::Read, it will only list those who are apart of the group conversation, if we allow for the event be emitted.