We are using boost::intrusive_ptr for a number of classes,
unfortunately by default we cannot see this in the Doxygen
collaboration diagrams.

However, we can work around this problem by making a dummy
namespace which we include in a header that is scanned by
Doxygen, but never included in LibreOffice. To be sure of this,
however, I have put #ifdef DOXYGEN_ONLY guards around the file
and defined this in the Doxygen config file.

Change-Id: I18d6956518a49e6006b64e2147023ec8266c8f5c