icon: update app icon

• GNOME 3.32 redesign of app icons. See Initiatives#2 (closed) for more info.

Our current icon (the one we use on the website for instance) is still nice, but it doesn't go that well with newest trends, and in particular the app icon redesign (cf. Jimmac's blog):

Also the max size we have available is apparently 256x256 which is not so nice if it needs to be printed (edit: actually @patdavid found the SVG source on the gimp-web repo!).

We also have this alternate version (the one used in icon themes, which has a SVG source so that's good), but I am not that fond of it (I mean, it's nice but it mostly looks like the previous one made flat):

And of course we have the symbolic version:

2 years ago, Aryeom designed this simple Wilber icon (SVG source of course), which we used for the printed t-shirt we offered to all present devs at LGM 2017:

Obviously the idea was more of a simple/elegant design for wear-printing and back then it had not been thought at all as a replacement for GIMP logo:

(as you can see, she also experimented with other designs, included color ones; hopefully she kept the SVG source even though we didn't use them in the end)

But now after re-reading @jimmac's blog post, I wonder if this could become a base for GIMP logo redesign. Simple shape, few colors, flat design, less details, no "hairlines"… The brush kind of disappeared here, but it could be brought back, though I wonder if it is necessary. Wilber is now known enough to be a brand by itself, I feel. People just recognize it.

@jimmac What do you think?

@mitch Opinion too?