title: BindableBase: Binding to nested property does not call setter
body: <p>I have got the following Models:</p>
  
  <pre><code>public class Car : BindableBase
  {
      private string _model;
      private string _wheels;
  
      public string Model
      {
          get { return _model; }
          set { SetProperty(ref _model, value); }
      }
  
      public string Wheels
      {
          get { return _wheels; }
          set { SetProperty(ref _wheels, value); }
      }
  }
  
  public class Customer : BindableBase
  {
      private Car _car;
  
      public Car Car
      {
          get { return _car; }
          set { SetProperty(ref _car, value); }
      }
  }
  </code></pre>
  
  <p>And the binding looks like this:</p>
  
  <pre><code>&lt;Page.Resources&gt;
      &lt;viewModels:CustomerViewModelLocator x:Key="PageViewModel" /&gt;
  &lt;/Page.Resources&gt;
  
  &lt;StackPanel&gt;
      &lt;TextBox Background="AliceBlue" Text="{Binding ViewModel.Car.Model, Mode=TwoWay, Source={StaticResource PageViewModel}}"&gt;&lt;/TextBox&gt;
      &lt;TextBox Background="AliceBlue" Text="{Binding ViewModel.Car.Wheels, Mode=TwoWay, Source={StaticResource PageViewModel}}"&gt;&lt;/TextBox&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p>I am using the ViewModelLocator pattern for my design time views and it looks good. But at runtime I don´t hit the setters of the models.</p>
  
  <p>What am I doing wrong?</p>
  
  <p>Trivial types within the Customer model would be set...</p>