Done based on the shape of the official AppImage logo, basing on SVG code of
the original icon for correct proportions, otherwise rewritten manually &
what inkscape does on the code.
The icons use a square background, like wanted for archives, packages,
compressed files, and disk images by the KDE HIG*, which also matches that
the appimage thumbnailer rom kio-extras will create a square preview, and
that other representations of applications are also square.