Fix Doxygen handling of XMLWRAPP_* macros.

The XMLWRAPP_DEPRECATED macro in particular wasn't parsed correctly by
Doxygen, resulting in ugly, confusing noise in the generated docs.

Also predefine XMLWRAPP_API to hide it from the docs.
Doxygen: create deprecated list.
Upgrade Doxyfile to current versions.

Just run doxygen -u on the file.
Don't hardcode font name.

This fixes the last remaining Doxygen warning.
Add PROJECT_BRIEF to Doxyfile.
Enable search in Doxygen docs.
Merge branch 'doxygen-fixes'

Various fixes and minor improvements to the Doxygen-generated documentation.