This is, as it turns out, the simple version of #166.
It changes the quit dialog to handle return by removing the
`.destructive` role from the `Quit` button.
When that role is set, the `keyboardShortcut(.defaultAction)` doesn't have an effect.
With the role removed, the dialog handles the return key to quit the
application
As I wrote in #166, I can understand if you don't want this change, but
I personally think it's nice to be able to only use the keyboard. iTerm2
also handles return like this in the "Are you sure?" dialog.
