Big Sur style app icon

[p-z-l]

While I'm working on the fancy mouse and menu detection thing, I think it's time to have a new app icon to match the overall design language of macOS Big Sur. Here is several candidates for the new rounded rectangle app icons, I'm not sure which one is better.

[viettrungphan]

woa, nice.

[viettrungphan]

Please create pull request, our designer really like it.

[p-z-l]

#118