Add support for XEP-0490: Message Displayed Synchronization

[wiktor-k]

Hi,

I'm using multiple clients and frequently have to double-click conversations to mark them as "read" but apparently there's a spec for doing that: https://xmpp.org/extensions/xep-0490.html

This was discussed also in https://gultsch.social/@daniel/113389447526952762

(FWIW I've looked through issues and couldn't find this being mentioned)

[Disaster2life]

I would very much like to see this, its very annoying to have re"display" messages that ive already seen on another client.

[sonicpp]

This is now supported by Gajim as well (2.1.0)