`xml:*` and `xlink:*` attributes are unconstrained strings in `mx::core`

[webern]

The generator passes attributes from xml.xsd and xlink.xsd through as std::string. Those schemas constrain the values, so invalid values are representable.

Five types need validators:

Attribute	Pattern	Schema
xml:lang	xs:language pattern, or empty	docs/xml.xsd:79
xml:space	default, preserve	docs/xml.xsd:89
xlink:type	simple, extended, title, resource, locator, arc	docs/xlink.xsd:32
xlink:show	new, replace, embed, other, none	docs/xlink.xsd:73
xlink:actuate	onLoad, onRequest, other, none	docs/xlink.xsd:85

Current representation: src/private/mx/core/generated/FormattedText.h:83,
AccidentalText.h:90, Link.h:57-61. 21 attribute sites total.

xlink:href, xlink:role, and @source are xs:anyURI, which accepts nearly any string.
std::string is correct there.

Related: #397.