Add consistent macOS app icon

[iLynxcat]

hey, are y'all interested in using a macOS icon that's consistent with the actual macOS icon style? I don't see any previous discussion on the repo about it so I'm opening an issue for it.

the easiest solution would be something like this:

App icon concept

App icon concept with official macOS icon design grid

I'll open a PR with this icon soon™, though would love some guidance on if (and if so, where) you'd like the source file to exist. based on the official macOS Production Templates for app icons, tossed it together in Sketch.