Merge Boostbook.

- Support some colour roles (thanks to Paul Bristow).
- Support assignment return types in doxygen2boostbook. Fixes #9241.
- Fix virtual methods in doxygen2boostbook.
- Add newlines to doxygen2boostbook to make the output a tad more readable.
- Improved last-revision handling.
- Support noexcept and constexpr in doxygen2boostbook.
- Improved boostbook for virtual and explicit methods from doxygen2boostbook.
- Support `language="c++"` attribute for `code` and `programlisting` elements.
- Use that to do syntax highlighting for doxygen2boostbook.


[SVN r86725]