Add quit menu item on macOS

[martinriviere]

Hi,

This fixes #652. I added the quit option in main menu, and current options are now in a separate Edit menu.
I also added productName property in package.json so Electron refers to it for the app name in the menus and everywhere using app.name (and no longer to the name property, which is in kebab case).

Thanks!

[whyboris]

Thank you so much!
And thanks for the productName improvement 😁
I'll try to find time this weekend to test it and merge it in 👍

[whyboris]

Thank you again @martinriviere for the improvement 🙇
Sorry it took so many months for me to get to it 😅 I was moving to a new house and just finally got back to VHA 🐌