title: adding conditional visibility to WPF control ToolTip
body: <p>i would like to make a textblock tooltip conditionally visible. </p>
  
  <p>i have the tooltip defined as:</p>
  
  <pre><code>&lt;TextBlock&gt;
      &lt;TextBlock.ToolTip&gt;
          &lt;Grid&gt;...&lt;/Grid&gt;
      &lt;/TextBlock.ToolTip&gt;
  &lt;/TextBlock&gt;
  </code></pre>
  
  <p>where would visibility property go in that definition? it doesn't seem to like any of my guesses.</p>
  
  <p>there are some suggestions of just working with grid visibility. the problem with that approach is making the grid invisible will still show empty tooltip box.. which is why i am trying to control tooltip visibility. </p>