title: Default command handler in Eclipse E4
body: <p>I am migrating an existing plugin to the Eclipse E4 API. </p>
  
  <p>We have defined some custom commands with default handlers, and I am trying to migrate these commands to E4, but I have not seen how to define a default handler for a command in E4. </p>
  
  <p>Has this functionality been removed or where can I find it?</p>