title: CanExecute methods not firing after moving to Catel 4.x
body: <p>Following up on <a href="https://stackoverflow.com/questions/35302365/canexecute-method-of-command-not-firing-after-update-to-catel-4-4-0/35311610#35311610">this</a> question and its answer (which did solve my problem), I have a couple of questions:</p>
  
  <ol>
  <li>Where do I retrieve the <code>ViewModelManager</code> to pass to the constructor of <code>RequeryAllTheThings</code>? Currently, I am using the <code>ViewModelBase.ViewModelManager</code> property in the first view model that loads, but this seems inelegant. I tried using the <code>ServiceLocator</code>, but the object it returned never managed any view models.</li>
  <li>The answer implies that there is an approach that will perform better, but I have not been able to find it in the documentation or on SO. Specifically, I need the CanExecute methods to fire for my commands when a control in a <code>DataGrid</code> changes state. What is the correct approach?</li>
  </ol>