A delegate used in RegisterForApplication and RegisterForWindow to provide callback hook for creating Touch-bar for given window or your application.
Inside the touch-bar creation hook you must:
1. Create instance of the Touch-Bar (optionally passing in item creation delegate in its constructor)
2. Set up default Touch Bar item identifiers with SetDefaultIdentifiers.
3. Either call SetTemplate items if item creation delegate was not use.
4. Optionally setup principal item with SetPrincipalItemIdentifier.
5. Return the newly created NSTouchBar.