Remove #include "config.h" from header files where possible

I noticed every change to config.h requires a recompile of most of the codebase.

This is a first shot at reducing the number of files affected by only including config.h in those header files that actually need it.
Touching config.h now results in recompilation of 675 files (which is still a lot but better than 783 files before this change).

Does this makes sense or am I overlooking anything? If yes, the same could be applied to .cpp files in future.