title: Tooltip style. Can't bind to datacontext
body: <p>i have one problem with tooltip. 
  I want to add templated tooltip to button with some information inside.</p>
  
  <p>Here the button with tooltip, which datacontext bounded to some viewmodel:</p>
  
  <pre><code>&lt;fluent:Button DataContext="{Binding NewConnections, Source={StaticResource Locator}}" Command="{Binding AddCloudStorageAccount}" Header="Add Account"&gt;
                          &lt;fluent:Button.LargeIcon&gt;
                              &lt;Image Source="pack://application:,,,/Icons;component/UI/v1/add_account.png" Width="48"/&gt;
                          &lt;/fluent:Button.LargeIcon&gt;
                          &lt;fluent:Button.ToolTip&gt;
                              &lt;ToolTip DataContext="{Binding UserInput.AddAccountsButtonInfo, Source={StaticResource Locator}}" Style="{StaticResource ButtonTooltip}"&gt;&lt;/ToolTip&gt;
                          &lt;/fluent:Button.ToolTip&gt;
                      &lt;/fluent:Button&gt;
  </code></pre>
  
  <p>Style:</p>
  
  <pre><code> &lt;Style TargetType="ToolTip" x:Key="ButtonTooltip"&gt;
      &lt;Setter Property="ContentTemplate"&gt;
          &lt;Setter.Value&gt;
              &lt;DataTemplate&gt;
                  &lt;Border  Background="LightYellow" BorderThickness="0.5" BorderBrush="Maroon"&gt;
                      &lt;StackPanel Orientation="Vertical" Margin="3"&gt;
                          &lt;TextBlock x:Name="_txtText" Text="{Binding Title}"&gt;&lt;/TextBlock&gt;
                          &lt;TextBlock x:Name="_txtDescription" Margin="0 10 0 0" Text="{Binding Description}"&gt;&lt;/TextBlock&gt;
                          &lt;TextBlock x:Name="_txtHotKeyDescription" Margin="0 10 0 0" Text="{Binding HotKeyDescription}"&gt;&lt;/TextBlock&gt;
                      &lt;/StackPanel&gt;
                  &lt;/Border&gt;
              &lt;/DataTemplate&gt;
          &lt;/Setter.Value&gt;
      &lt;/Setter&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>I set some breakpoints to see if viewmodel is accessed. And it's ok. But properties like Title not accessed at all and i see only empty rectangle without any text</p>
  
  <p>Do someone have some ideas?</p>