deltachat.h uses `@defgroup` commands to create topics
for groups of constants. Prior to Doxygen 1.9.8
defining a group created a "module"
and all constants were visible from the modules.html page.
In Doxygen 1.9.8 "modules" were renamed into "topics"
as C++20 modules have taken their place,
so Delta Chat documentation does not have modules
in Doxygen sense anymore.
The change is to replace "modules.html" with "topics.html"
in the DoxygenLayout.xml.
See <https://www.doxygen.nl/manual/grouping.html> for
Doxygen documentation about groups and their relation to topics.
`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
