Replace custom inset title bar (macOS)

In Atom `1.16` (currently [Beta](https://atom.io/beta)), there will be an option to use a custom **inset** title bar: https://github.com/atom/atom/pull/13616. It has a bit more padding around the traffic lights:


Not that useful, but it would be neat if the tool-bar would replace that title bar. Something like:


Not sure if it should be opt-in or opt-out or all the time a user would use a combination of: custom-inset title bar + full width top tool bar.

Anyways, I can try to make a PR in case there is interest.

Start by reviewing Atom 1.16's custom inset title-bar option and the linked atom/atom#13616 pull request, then inspect this package's toolbar layout and styling entry points. Before implementation, resolve whether the behavior is opt-in, opt-out, or always enabled; done should include an agreed appearance and verified compatibility with the inset title bar.

Written by the indexing model from the issue text.