Inner class that uses the type-safe enum pattern for the 10 DTD types:
Attribute.TYPE.CDATA
Attribute.TYPE.ID
Attribute.TYPE.IDREF
Attribute.TYPE.IDREFS
Attribute.TYPE.NMTOKEN
Attribute.TYPE.NMTOKENS
Attribute.TYPE.NOTATION
Attribute.TYPE.ENTITY
Attribute.TYPE.ENTITIES
Attribute.TYPE.ENUMERATION
Attribute.TYPE.UNDECLARED