Fixes several files incorrectly stated a different license from the actual project, as well as the copyright headers included in all files. This change has no effect on the licensing terms, it should clear up a bit of confusion by contributors. Plus the files get a bit smaller, and we have less duplicated information across the entire project.
Overall the project is GPLv2 if not built with Qt, and GPLv3 if it is built with Qt. There are no parts licensed under a different license, all have been adapted from other compatible licenses into GPLv2 or GPLv3.
Adds supports for running clang-tidy from within CMake, if the Clang toolset was found. This feature is experimental, but should work on many compilers, as it relies on generated compile_commands.json which are fully generated by the clang subproject. Using clang-tidy we can find hidden bugs that other static analyzers do not detect, or compilers don't even bother throwing an error for.
