Use bundle as a means to provide Mac APIs to Catalyst app.
In order to cross the @objc wall set by the Mac Bundle mechanism,
Swift structures cannot be used directly and must be bridged
through ObjC facades.
Create NSMenu in MVVM style and install it on app launch. Make
sure to do it in AppDelegate.applicationDidFinishLaunching(),
because doing it as early as in PassepartoutApp.init() would
crash Mac code.
Use .representedObject to own view models.
With menu in place, app can be sent to background when main window
is closed. Requires multiple documents support for app not to die
instantly.
