title: Pattern for binding commands on a child ViewModel from a parent menu
body: <p>I'm creating a WPF MVVM app using Caliburn Micro.  I have a set of buttons in a menu (Ribbon) that live in the view for my shell view model, which is a ScreenConductor.  Based on the currently active Screen view model, I would like to have the ribbon buttons be disabled/enabled if they are available for use with the active Screen, and call actions or commands on the active Screen.</p>
  
  <p>This seems like a common scenario.  Is there a pattern for creating this behavior?</p>