Replace the flatpak icon.

As vanilla, Pd-extended, Pd-l2ork, and Purr Data are all on Flathub, we
need something that looks different from all the other flavors. This
"small_cat within the Pd-extended icon" icon will do the trick for
now. I believe it was @joe-rouen who originally came up with the idea,
but I've since lost his design, and so I created my own svg instead.

I'm obviously no graphic artist. If you are, and you don't like my
design, send me yours in a PR on GitHub.

Fixes https://github.com/agraef/purr-data/issues/26