add copyright information to every touched source file

I don't want to claim copyright on this changes, so I put this commit and its changes under CC0 (https://creativecommons.org/publicdomain/zero/1.0/).
It does not touch the GPL license, which are all modified files under. This step simply prevents a decleration of copyrights notes I would put these files under with exactly this change.

todo:
branding.h doesn't have a proper license decleration
*.rc files are marked as generated