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-debug/src/main/java/org/jivesoftware/smackx/debugger/EnhancedDebugger.java

@@ -943,6 +943,11 @@ public class EnhancedDebugger extends SmackDebugger {
         public String toString() {
             return toXML((XmlEnvironment) null);
         }
+
+        @Override
+        public String getElementName() {
+            return null;
+        }
     }
 
     /**