Apply builder pattern to form fields and replace getVariable() with
getFieldName(). Refer to the field name as "field name" instead of
"variable" everyone, just as XEP-0004 does.
Improve the high-level form API: introduce FilledForm and FillableForm
which perform stronger validation and consistency checks.
Also add FormFieldRegistry to enable processing of 'submit' forms
where the form field types are omitted.
Smack also now does omit the form field type declaration on 'submit'
type forms, as it is allowed by XEP-0004.
- 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.