doxygen: improve #include information display based on real-world use.

doxygen: improve #include information display based on real-world use.

 * In cases where Doxygen spots the namespace first in a *.cpp file (and
   later in a header) but the namespace is still contained in a single
   header, the include informations was improperly put locally into
   members, instead of globally. Now the information is propagated
   upwards from the members to ensure correctness. This needs also
   special handling for namespaces that are otherwise empty, where
   there's nothing that could propagate the info upwards.
 * Complex namespaces (namespaces containing other namespaces or
   classes) no longer have the include information global even though
   the non-class/namespace members are in the same file. This would be
   misleading since inner namespaces and classes are often in different
   files.