Unsurprisingly doxygen does not support reading a idl_chapter_refs.txt
file like autodoc does.  And it looks like doxygen tag files can not
be used to do this, as they link only in the "other" direction.

But with a bit of hackery we can generate a "dummy" IDL input file, with
emtpy definitions for those types that have DevGuide links.
Doxygen will nicely combine documentation from multiple definitions of a
type, and fortunately does only put in references to the first file that
defines a type, so hopefully adding the generated file as the last input
file is suffficient to make things work nicely.

The generated file is committed to git because there is a bit of a
problem with finding out what sort of entity a name like
"com.sun.star.Foo" refers to, which needs to be in the doxygen input.

Change-Id: I793bc59c8acfecf460e4addea7838c76a3ede77e