Make appdata.xml conformant with spec, choose category, follow recommendations for filenames

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