instead of using a PacketListener, which means that the user has to
downcast the Packet to Message, we now use a Listener which callback
parameter is already Message/Presence.
It is necessary to introduce MessageListener and PresenceListener, which
are interfaces that have a callback for Message/Presence instead of
Packet. The 'old' MessageListener is renamed to ChatMessageListener.
Use Generics in ConnectionDetachedPacketCollector.
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