Add a link to the unused animated gem-click game

from the chapter 18 index (next to the other versions of the game), even though the files are in chapter 19

ch18-interaction-files/index.html

@@ -140,6 +140,16 @@ <h2 id="examples">Figures and Examples<a href="#examples" class="hash-link" aria
             <a class="filename" title="view source on GitHub" href="https://github.com/oreillymedia/Using_SVG/tree/master/ch18-interaction-files/gem-click-game-misses.png">gem-click-game-misses.png</a></li>
         </ul>
       </dd>
+      <dt>A variation on the gem-click game with moving gems
+          (created but not used for the <a href="../ch19-animation-files/">Chapter 19</a> section on scripted animation)</dt>
+      <dd>
+        <ul>
+          <li><a href="../ch19-animation-files/gem-click-game-motion.svg">Live SVG file</a>,
+            <a class="filename" title="view source on GitHub" href="https://github.com/oreillymedia/Using_SVG/tree/master/ch19-animation-files/gem-click-game-motion.svg">ch19-animation-files/gem-click-game-motion.svg</a></li>
+          <li><a href="../ch19-animation-files/gem-click-game-motion.js">Raw JS file</a>,
+            <a class="filename" title="view source on GitHub" href="https://github.com/oreillymedia/Using_SVG/tree/master/ch19-animation-files/gem-click-game-motion.js">ch19-animation-files/gem-click-game-motion.js</a></li>
+        </ul>
+      </dd>
       <dt><span class="label">Example 18-7. </span>Making links behave like buttons, with JavaScript event handlers</dt>
       <dd>
         <ul>
@@ -184,4 +194,4 @@ <h2 id="extras">Online Extras<a href="#extras" class="hash-link" aria-label="Per
       View the <a href="https://github.com/oreillymedia/Using_SVG/">GitHub repository</a> to download the files, suggest a correction, or see when a page was last updated.</p>
   </footer>
 </body>
-</html>
+</html>