calamares.desktop

* Fix spelling mistake in kdsingleapplicationguard.cpp

s/emty/empty/g.

* Add fdo keywords

Add freedesktop.org keywords to desktop entry.

This allows users to search for the app in various desktop environments.

For more details, see:
    https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s05.html,
    https://bugs.debian.org/693918, and
    https://wiki.gnome.org/Initiatives/GnomeGoals/DesktopFileKeywords for