Make appdata.xml and desktop file conformant with specs and recommendations
cf.
- https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps
- https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent
- https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html
- https://standards.freedesktop.org/menu-spec/latest/apa.html
- https://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#desktop-file-id