ExtensionElement is now a marker interface that requires all
implementation non-abstract classes to carry a static final QNAME
field (of type QName). This is verified by a new unit test.
Also FullyQualifiedElement is renamed to simply XmlElement. XmlElement
is used over ExtensionElement when implementing classes do not
statically know the qualified name of the XML elements they
represent. In general, XmlElement should be used sparingly, and every
XML element should be modeled by its own Java class (implementing
ExtensionElement).
If it is announced as feature, entities sending fastened messages
expect the recipient to react somehow on those. And this is not the
case if this is just enabled in Smack.
Hence we disable it per default and require smack users to explicitly
enable it after they have setup the according stanza listeners.