Add SPDX markers for a bunch of remaining trivial files

Down to only two missing files for full REUSE compliance now.
2 jobs for master in 10 minutes and 55 seconds (queued for 14 minutes and 48 seconds)
Status Job ID Name Coverage
  Build
passed #93255
build_clazy_clang_tidy

00:08:22

passed #93254
linux kf5-qt5 qt5.15

00:02:33

71.0%