More fixes for issue 57.

- move the selection highlight code to the Text mode.
- store $selectionOccurrences in the editor session.
- add option to toggle selection highlighting. In demo page as well.

index.html

@@ -108,6 +108,10 @@
         <label for="show_print_margin">Show Print Margin</label>
         <input type="checkbox" id="show_print_margin" checked>
       </td>
+      <td align="right">
+        <label for="highlight_selected_word">Highlight selected word</label>
+        <input type="checkbox" id="highlight_selected_word" checked>
+      </td>
       </tr>
     </table>