title: Place holder or watermark in TextBox windows 8
body: <p>I want to show a placeholder text in <code>TextBox</code> when user hasn't typed anything and <code>TextBox</code> is idle.<br><br>
  In Andriod it can be done using  <code>android:hint="some Text"</code><br>
  In iPhone it can be done as <code>textFild.placeholder = "some text";</code></p>
  
  <p>How can I do it in windows 8 metro apps?</p>
  
  <p>Thanks</p>