Detect room disconnections

Scenario:

The room crashes, and restarts. Poezio is not notified of this event, and is not in the room anymore (because we didn’t rejoin), so we do not receive any more presence or message from the room or its occupants, and we never know we’re not in the room anymore.

One possible implementation: ping our own user (test@kikoo.louiz.org/louiz’) every X seconds. If we receive an error, we consider we are offline and we try to auto-join or set a offline color on that tab (depending on the configuration).