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-extensions/src/main/java/org/jivesoftware/smackx/xdata/FormField.java

@@ -417,7 +417,7 @@ public final class FormField implements FullyQualifiedElement {
         } else {
             buf.rightAngleBracket();
 
-            buf.append(formFieldChildElements, enclosingNamespace);
+            buf.append(formFieldChildElements);
 
             buf.closeElement(this);
         }

smack-extensions/src/main/java/org/jivesoftware/smackx/xdata/packet/DataForm.java

@@ -325,15 +325,10 @@ public class DataForm implements ExtensionElement {
 
     @Override
     public XmlStringBuilder toXML(XmlEnvironment xmlEnvironment) {
-        XmlStringBuilder buf = new XmlStringBuilder(this);
+        XmlStringBuilder buf = new XmlStringBuilder(this, xmlEnvironment);
         buf.attribute("type", getType());
         buf.rightAngleBracket();
 
-        XmlEnvironment dataFormxmlEnvironment = XmlEnvironment.builder()
-                        .withNamespace(NAMESPACE)
-                        .withNext(xmlEnvironment)
-                        .build();
-
         buf.optElement("title", getTitle());
         for (String instruction : getInstructions()) {
             buf.element("instructions", instruction);
@@ -347,7 +342,7 @@ public class DataForm implements ExtensionElement {
             buf.append(item.toXML());
         }
         // Add all form fields.
-        buf.append(getFields(), dataFormxmlEnvironment);
+        buf.append(getFields());
         for (Element element : extensionElements) {
             buf.append(element.toXML());
         }