Define \ditamap and \topicref in qdoc for creating ditamaps when generating DITA XML output

I think what we need a new page command: \ditamap
and a \topicref \endtopicref command pair.

/*!
\ditamap
\title Table of Contents for Creator

\topicref

\endtopicref
\endtopicref

*/

We also need \mapref, which in DITA is a <topicref format="ditamap">. The \mapref will be used to create a ditamap of ditamaps. The vision here is to let each repository contain one \ditamap qdoc file that specifies the table of contents for the repository. There can be multiple \ditamap files in a repository.

There will also be a \ditamap somewhere that is "The Lord of the ditamaps," which specifies the ordering of all the repository ditamaps in an all-encompassing ditamap. This master ditamap will be used to create the <toc> element in the qhp file created by qdoc. There can be multiple master ditamaps as well, but the one qdoc uses will be specified in a new "toc" variable in the qdocconf file.

For the DITA-impaired, \toc, \tocentry, and \tocref can be used in qdoc files instead of \ditamap, \topicref, and \mapref respectively.