Add XmlStringBuilder.emptyElement(Enum)
Use XmlStringBuilder in AdHocCommandData

also remove 'lang' attribute, since it belongs into IQ, see XEP-50 3.7:

"The requester SHOULD provide its locale information using the "xml:lang"
attribute on either the <iq/> (RECOMMENDED) or <command/> element."
Remove XmlLangStanza

since all stanza types could have 'xml:lang' defined as top-level
stream element attribute.