Enable PacketExtensions for IQs

This is actually only part one, i.e. with this commit if the user adds a
PacketExtension to an IQ it will be included in IQ.toXml(). Which was
previously only the case if the IQ subclass explicitly included packet
extensions.

The second part of the change is to change the IQ provider, so that
packet extensions are automatically parsed.

Cases where PacketExtensions are used for Message and IQ are slightly
changed. The IQ sublcass now only has a field with this
PacketExtension (see for example
bytestreams.ibb.packet.DataPacketExtension).

Also changed hoxt API: Removed unnecessary indirection and made the
API more Smack idiomatic.

smack-extensions/src/main/java/org/jivesoftware/smackx/time/packet/Time.java

@@ -43,6 +43,7 @@ public class Time extends IQ {
     private String tzo;
 
     public Time() {
+        super(ELEMENT, NAMESPACE);
         setType(Type.get);
     }
 
@@ -53,6 +54,7 @@ public class Time extends IQ {
      * @param cal the time value.
      */
     public Time(Calendar cal) {
+        super(ELEMENT, NAMESPACE);
         tzo = XmppDateTime.asString(cal.getTimeZone());
         // Convert local time to the UTC time.
         utc = XmppDateTime.formatXEP0082Date(cal.getTime());
@@ -128,14 +130,17 @@ public class Time extends IQ {
         return time;
     }
 
-    public String getChildElementXML() {
-        StringBuilder buf = new StringBuilder();
-        buf.append("<" + ELEMENT +  " xmlns='" + NAMESPACE + "'>");
+    @Override
+    protected IQChildElementXmlStringBuilder getIQChildElementBuilder(IQChildElementXmlStringBuilder buf) {
+        buf.rightAngleBracket();
+
         if (utc != null) {
             buf.append("<utc>").append(utc).append("</utc>");
             buf.append("<tzo>").append(tzo).append("</tzo>");
+        } else {
+            buf.setEmptyElement();
         }
-        buf.append("</" + ELEMENT + ">");
-        return buf.toString();
+
+        return buf;
     }
 }