Add window menu

[felixrieseberg]

The app currently doesn't have a window menu. You might see one when you run the app in developer mode, but said window menu will no longer be there once you compile the application (that's a feature implemented by the electron command).

[delvedor]

We must figure out if we need a window menu, seen the current features it might be useless.

[felixrieseberg]

@delvedor You do want Alt + F4, Copy, Paste, and all the other basic commands to work though, right 😉 At least on macOS and Linux we'll need it to enable those common features, on Windows I'm down for a custom built one (either the same thing I did over at Slack or alternatively stealing from taking inspiration from GitHub Desktop with a completely custom implementation).