GUACAMOLE-346: Display "seek in progress" notification when seeking in playback example.

doc/guacamole-playback-example/src/main/webapp/index.html

@@ -31,7 +31,13 @@
         <div class="player">
 
             <!-- Player display -->
-            <div id="display"></div>
+            <div id="display">
+                <div class="notification-container">
+                    <div class="seek-notification">
+                        <p>Seek in progress... Click "play" to cancel.</p>
+                    </div>
+                </div>
+            </div>
 
             <!-- Player controls -->
             <div class="controls">