This is just a cleanup of include statements.
Previously there were some includes that weren't used at all and others that were used only because they transitively included other files.
By removing unnecessary includes and by moving useful ones from header files into cpp files, this change dimishes the amount of preprocessed code that is fed to the compiler, perhaps improving the parsing (useful for IDEs as well) and compilation time.

These changes are meant not to have any functional impact on the code itself.