Add support for "Caps Optimizations"

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.

smack-im/src/test/java/org/jivesoftware/smack/roster/SubscriptionPreApprovalTest.java

@@ -73,7 +73,7 @@ public class SubscriptionPreApprovalTest extends InitSmackIm {
     @Test(expected=FeatureNotSupportedException.class)
     public void testPreApprovalNotSupported() throws Throwable {
         final Jid contactJID = JidCreate.from("preapproval@example.com");
-        roster.preApprove(contactJID);
+        roster.preApprove(contactJID.asBareJid());
     }
 
     @Test