Complete the DTD attribute handling found in the audit:
- Default and #FIXED attribute values are applied to the parsed tree.
parseApplyingInternalDTDDefaults returns a tree where every element
carries the attributes its <!ATTLIST> defaults, so an editor sees the
same effective attributes a validating processor would.
- NMTOKEN, NMTOKENS, ENTITY, ENTITIES are validated: NMTOKEN(S) must be
XML name tokens, ENTITY/ENTITIES must be XML names. Previously all fell
through to unconstrained CDATA.
- DTD default values are normalized per XML 1.0: character and predefined
entity references are expanded, literal whitespace becomes a space, and
non-CDATA values are collapsed and trimmed.
ENTITY/ENTITIES check name syntax only; that the named entity is a
declared unparsed entity is not verified, since NDATA entity tracking is
not yet modeled.
Closes#73
