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).
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