Rework XML Element hierarchy and XmlStringBuilder

- Reduce the amount of types that are subtypes of NamedElement. See
javadoc of NamedElement for rationale.
- Work more with XmlEnvironment in XmlStringBuilder.
- Some minor changes to XmlStringBuilder API.

smack-legacy/src/main/java/org/jivesoftware/smackx/workgroup/packet/AgentStatusRequest.java

@@ -65,14 +65,6 @@ public class AgentStatusRequest extends IQ {
         return Collections.unmodifiableSet(agents);
     }
 
-    public String getElementName() {
-        return ELEMENT_NAME;
-    }
-
-    public String getNamespace() {
-        return NAMESPACE;
-    }
-
     @Override
     protected IQChildElementXmlStringBuilder getIQChildElementBuilder(IQChildElementXmlStringBuilder buf) {
         buf.rightAngleBracket();