title: ActiveViewModels in Catel
body: <p>currently I have a solution with multiple ViewModels - "normal" and "nested" ones.
  Every time I create a new ViewModel instance, I can see the new instance in the ViewModelManager.ActiveViewModels. So far so good.<br>
  Unfortunately I also can see "old" ViewModel instances there, in fact, instances that are not in use but still remain in this "Cache".<br><br>
  My first question is now, what do I have to do for disposing this "old" ViewModels? <br>
  And an additional question (follow-up)...: which is the recommended usage of ViewModel instantiation to avoid this behavior?<br><br>
  Thanks in advance</p>