`DoxygenLayout.xml` is picked up by doxygen automatically (as `Doxyfile`)
and its template can be generated by `doxygen -l DoxygenLayout.xml`.
after that, the following tweaks were done:
- remove all xml-parts but header (other defaults are fine so far)
- rename "Modules" header to "Constants" (there are no modules :)
- remove useless subitems from "Classes", eg. by name only shows as "d"
- reorder tabs so that "Classes" becomes the first one
