add app icon for macos

[mariodian]

I propose this small change to the app's icon so it's more in line with Apple's design guidelines (added padding and shadow).

I left the original icon intact so the change doesn't affect Linux and Windows users.

I believe changes to window and tray in electron/index.cjs aren't necessary because:

1. there's no window icon in any MacOS (only OSX which is fairly old)
2. the app has no ability to be displayed in top menu bar currently (I assume that's what electron's Tray() is in MacOS)

If you want me to add the icon to window and tray though, I'm happy to do so.

[ztjhz]

Nice work! Just need a new line at the end of the package.json file