Add support for XEP-0490: Message Displayed Synchronization
Description of the new feature
This is a new XEP that allows you sync the displayed state of your chats between multiple clients. So if you read a number of messages on your phone, then when you switch to your desktop those messages would be considered read as well XEP-0490: Message Displayed Synchronization
It is preferred to implement this after !960 gets merged. Otherwise when you select a chat all the messages in it would be marked as read in all the clients implementing mds.