[IMP] website: add key in ir.ui.view list view if multi-websites

With multi-website support, the key field is more important than ever as it
kind of replace the xml_id behavior.

Thus, on the ir.ui.view list view it is usefull to see the key.

closes odoo/odoo#29419

addons/website/views/website_views.xml

@@ -292,6 +292,9 @@
                 <field name="name" position="after">
                     <field name="website_id" groups="website.group_multi_website"/>
                 </field>
+                <field name="xml_id" position="before">
+                    <field name="key" groups="website.group_multi_website"/>
+                </field>
             </field>
         </record>