Properly escape Bookmarks and FormField XML

by using XmlStringBuilder. Fixes SMACK-577

Also extend LazyStringBuilder with a cache. And extend XmlStringBuilder
with some more convenience methods.

Move the ELEMENT and NAMESPACE definition from Form to DataForm, where
it belongs.

smack-extensions/src/main/java/org/jivesoftware/smackx/iqprivate/packet/PrivateData.java

@@ -45,5 +45,5 @@ public interface PrivateData {
      *
      * @return the private data as XML.
      */
-    public String toXML();
+    public CharSequence toXML();
 }