Smack's previous entity caps implementation assumed that an entity lost
its entity caps feature as soon as a presence without caps from that
entity was received. But according to XEP-0115 § 8.4, this is a
perfectly normal optimization technique. We now reset the caps state
after an available presence becomes unavailable.
Also introduce PresenceEventListener, which is required for this
feature.
Also make Roster.preApprove() take a BareJid as argument.
Fixes SMACK-723.
Florian Schmaus