maintain appstream and desktop files

[merge]

maintain the .desktop files for ts_calibrate and ts_test_mt. this would be the appropriate place for those, see https://www.freedesktop.org/wiki/Howto_desktop_files/

the same goes for appstream xml files, see https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html

[merge]

or probably better: only a .desktop file that says we don't need an appstream file.