[FIX][website_mail_snippet_table_edit] Compatibilize with website_less.

Without this patch, `website_less` disables this addon.

With this patch, it improves it.

Win-win! 😊

website_mail_snippet_table_edit/README.rst

@@ -36,8 +36,9 @@ To use this module, you need to:
 Known issues / Roadmap
 ======================
 
-* You cannot drag & drop table rows among tables, and that's because of the
-  inner design of Odoo's drag & drop engine, that does not allow that easily.
+* You cannot drag & drop table rows among tables, unless you install
+  `website_less <https://www.odoo.com/apps/modules/8.0/website_less/>`_.
+* New mail editor engine in Odoo 9.0+ makes this module unnecessary there.
 
 Bug Tracker
 ===========

website_mail_snippet_table_edit/views/snippets.xml

@@ -9,8 +9,8 @@
               inherit_id="website_mail.email_designer_snippets">
         <xpath expr="//div[@id='snippet_options']">
             <div data-snippet-option-id='tr_editor'
-                data-selector="[data-oe-field='body_html'] tr">
-            </div>
+                 data-selector-children="[data-oe-field='body_html'] tbody, [data-oe-field='body_html'] thead, [data-oe-field='body_html'] tfoot"
+                 data-selector="[data-oe-field='body_html'] tr"/>
         </xpath>
     </template>