Fix linux tab margin

[c0b41]

#433

[albinekb]

This will probably need to happen on windows too, http://electron.atom.io/docs/api/browser-window/

Maybe it's better to only set titleBarStyle on macOS and then use that to decide the margin?
https://github.com/zeit/hyperterm/blob/master/app/index.js#L52

[rauchg]

It should be 76 if it's explicitly darwin as opposed to non-linux