Revert "remove editor div in filelist", add "is deprecated" comment

This reverts commit 64d09452.

apps/files/templates/index.php

@@ -101,6 +101,7 @@
 		<?php print_unescaped($_['fileList']); ?>
 	</tbody>
 </table>
+<div id="editor"></div><!-- FIXME Do not use this div in your app! It is deprecated and will be removed in the future! -->
 <div id="uploadsize-message" title="<?php p($l->t('Upload too large'))?>">
 	<p>
 	<?php p($l->t('The files you are trying to upload exceed the maximum size for file uploads on this server.'));?>