title: UWP TextBox.Text binding not updating when using PlaceholderText
body: <p>I've got some text boxes for which i wanted to set the <code>PlaceholderText</code> property. The text of each box is bound to a property of the underlying view model. Now when setting the placeholder in the XAML like that</p>
  
  <pre><code>&lt;TextBox PlaceholderText="Placeholder" Text={Binding PropertyName} /&gt;
  </code></pre>
  
  <p>i noticed, that the view model's properties are not updated anymore when the text box loses focus.
  Whereas without placeholder the binding works just fine.</p>
  
  <p>Is this behaviour intended and if are there any workarounds, or do i have to stick to a classic <code>TextBlock</code> that describes the intended input each box?</p>
  
  <p><strong>Edit:</strong> The property does implement <code>INotifyPropertyChanged</code> and the binding is updated in the view model when no placeholder is set.</p>