Changes to Bookmap for DITA 2.0

[Chris Nitchie <chris.nitchie@oberontech.com>]

Arguably the biggest problem with the current BookMap design is the inability to use keydef and other topicref specializations at the top level. This makes it difficult to define keys, and impossible to assign <ditavalref> references. To address this issue in DITA 2.0 while keeping the BookMap grammar backwards-compatible with existing documents, I suggest we

- Allow <topicref> and its specializations before <frontmatter>
- Allow <topicref> and its specializations in the body between <frontmatter> and <backmatter>.

I think this would be all that’s needed to allow BookMap to be more modular and flexible.

Chris