Add in and out afk events.
The actual event is generic: https://wow.gamepedia.com/PLAYER_FLAGS_CHANGED
After the event is fired, we need to check the status with https://wow.gamepedia.com/API_UnitIsAFK and https://wow.gamepedia.com/API_UnitIsDND

To be able to manage this, a new king of virtual event is needed. It requires a few changes in the core of the addon (right now the events are referenced by their ingame names)