Draft: Update App-Icons for macOS to follow Apple's Design Guidelines

Current App-Icons doesn't follow current design guidelines for macOS (which are described here, templates for icons available here), and while there are a new proposed icon on !733 wouldn't it be better to use slightly modified icon from iOS version? Apple themselves encourage design continuity, which makes using modified iOS icon a far better idea. Not sure what to do with Xmas icon, since iOS app doesn't have it