In the WWDC slides and the sample code, Apple touts a buildCommands method on UIResponder for menus. However, this was deprecated with beta 1. Instead, we should use buildMenu(with:). This currently lacks documentation though.
It seems, currently, the best way to modify the Menubar (or, MainMenu) is by using storyboards and dropping a Main Menu into your storyboard. This new scene will be just like the default menu, but you can edit it.
