Make the DITA XML generator output <apiRelation> elements where a type name is now output as plain text.

There are several places in the DITA XML output for a class reference where type names are output as plain text. In the HTML output generator, these type names are output as <xref> elements. Change the DITA XML output generator to output <apiRelation> elements for these plain text type names wherever the HTML output generator outputs <xref> elements. Note that in DITA XML, the <apiRelation> element inherits the <xref> element, but the DITA validator does not allow using an <xref> where the schema requires an <apiRelation>.