Reuse compliance

This MR is a fairly mechanical find & "fix" exercise to see what's needed to get to REUSE compliance for Keysmith. You can verify compliance using the reuse commandline tool:

pip3 install --user reuse
~/.local/bin/reuse

Discussion points

• I've assumed that dates of committing/author ship also indicate year of publication. That happens to be true in CI terms, but of course this does not correspond 1:1 to actual releases.
• It may be preferable to have the licensing/copyright statements for the desktop file, appdata, and icon in separate *.license files: does (automated) tooling preserve the comments?