title: Get Selected Items collection from a ReactiveList using ReactiveUI
body: <p>I have bound a ReactiveList Orders to a datagrid in a wpf.
  I succeeded in obtaining a single selected order by following syntax</p>
  
  <pre><code>private Order _selectedOrder;
  public Order SelectedOrder
  {
      get { return _selectedOrder; }
      set { this.RaiseAndSetIfChanged(ref _selectedOrder, value); }
  }
  this.WhenAnyValue(x =&gt; x.Orders.ItemChanged).Select(x =&gt; _selectedOrder = ((Order)x));
  </code></pre>
  
  <p>But if I multi select the orders in datagrid how do I get the selected items?</p>