Message dialogs should hide unused window controls

MessageBox and MessageDialog currently display as a standard desktop window on macOS, which leaves unused native traffic-light window controls visible in the title bar. For alert-style message dialogs, users expect only the dialog action buttons, such as OK, Cancel, or Alternate, to be visible.

Expected behaviour: message dialogs should use alert-style window chrome where possible so inactive minimise/zoom controls are hidden or not shown.