title: ShowDuration attribute cannot resolve in tooltip at silverlight 4
body: <p>i want use to ShowDuration attribute in a tooltip at silverlight project. but ShowDuration not exist in ToolTip control</p>
  
  <p>for example:</p>
  
  <pre><code>&lt;Button x:Name="btnAppUserRoleAdd" Style="{StaticResource GlassButton}" Content="button" Width="100" Height="36" Margin="0 0 40 0"&gt;
                  &lt;ToolTipService.ToolTip&gt;
                      &lt;ToolTip ShowDuration="10000" Template="{StaticResource ToolTipTemplate}" HorizontalOffset="2" VerticalOffset="5"&gt;
                          &lt;ToolTip.Content&gt;
                              &lt;TextBlock Text="any test" 
                                         TextWrapping="Wrap" Style="{StaticResource ButtonTooltipFontStyle}" /&gt;
                          &lt;/ToolTip.Content&gt;
                      &lt;/ToolTip&gt;
                  &lt;/ToolTipService.ToolTip&gt;
  </code></pre>
  
  <p></p>
  
  <p>"ShowDuration="10000" this attribute have not known by intellisence in VS2010 and that say: cannot resolve symbol ShowDuration</p>
  
  <p>my tooltip without ShowDuration working properly and not problem. but i want use this attribute. of course, i use tooltip class in code (c#) but problem not solved.</p>
  
  <p>please help me
  i use silverlight4, visual studio 2010.</p>