title: Cannot add handler in eclipse e4 application model since upgrading to oxygen
body: <p>I am working on an Eclipse e4 RCP application with multiple fragments/plug-ins. 
  I upgraded from Neon to Oxygen and now I have the problem, that I can't add any handler or command in my fragments/plug-ins.
  When trying the same in the main application.e4xmi it's no problem, but the other e4xmi-models don't have the context-menu item to add a handler to the model-fragment (also not possible in the details view of the fragment --> list to select the child is empty). 
  All my previous handlers and command are there.</p>
  
  <p>Am I missing some plug-in oder dependency? I only upgraded to Oxygen.</p>
  
  <hr>
  
  <p>Added: To clarify, I did not upgrade my existing eclipse to oxygen. I installed a new Oxygen RCP-Client. Did I forget to add anything to make this work again?</p>
  
  <hr>
  
  <p>Added Nr2:
  As greg-449 proposed using the "<strong>xpath:/</strong>"-Expression instead of the full ID of the main e4xmi Model as a reference for the "Extenden Element ID" works when trying to reference elements on the first level of the model like commands, handerls and key bindings. 
  But whe trying to use more complex expressions like for referencing the perspective stack or a toolbar this method doesn't work any more.</p>