Ability to mark messages as unread

In a similar vein to email, it would be good to mark a message you plan to go back to as unread so you can focus on other things. Currently the way people work around this is read messages from their notifications and avoid clicking on the room to clear the unread marker. Obviously, this means the user cannot interact with the room in any way in the meantime, aside from it being intuitive.

The simple solution I would propose is a room-account-data item, which simply lists event_ids in an array. When marking a message as read, the eventID is entered into the array. Clients should take this into account and increase the rooms unread counter by this number, as well as highlighting the message in some way.

The eventID should be removed from the array once the user has indicated that the message has been read.

Start by reviewing the proposed room-account-data item and how Matrix currently represents read messages and unread counters. Define the protocol and client behavior for adding and removing event IDs, including how clients highlight messages and update room counts; done means the behavior is specified consistently for all clients.