Since 35200a29ea we build complete installers using CMake, meaning that there's no need for BuildUtil anymore.
MSBuild projects that are not migrated to CMake yet are kept for reference.
This commit also updates BUILD_WINDOWS.md so that it mentions Visual Studio 2019 instead of 2017.
BUILD_WINDOWS.TXT, BUILD_UNIX.TXT removed from root folder
src/BUILD_WINDOWS.TXT, src/BUILD_UNIX.TXT converted to "md"
BUILD_WINDOWS.TXT, BUILD_UNIX.TXT are now hyperlinks in README.md