macOS utilities for creating .icns file are non-deterministic and
the resulting file has different checksum each time the file is
re-generated. We don't want this to influence reproducibility of
builds in the future.
Windows is missing necessary native utilities for generating .ico
files out of .png sources.
It's also easier to just copy generated file during snapshot build.
SVG icon as a source for other sizes, PNG icon 1024x1024 as source for
generating .icns file, and small 16x16 PNG icon.
Background circle uses size and colours recommended by Gnome HIG.
