Partial implementation of XEP-0333.
Gajim sends a
displayed chat marker if we have pending events and:
- we open a new message control (
read_queue) / set it to active (switching tabs,
- we scroll to the end of a message control (
- we open a minimized group chat (
Gajim removes unread events when we receive a
displayed marker for the last message.
Fixes #8163 (closed)