[REF] web: ListRenderer: extract function for inheritance

This commit defines a "displayOptionalFields" getter that can be
overriden to force the presence of the optional column dropdown.
For instance, this is necessary in enterprise to add an item to
promote Studio.

Part-of: odoo/odoo#78221

addons/web/static/src/views/list/list_renderer.js

@@ -441,6 +441,10 @@ export class ListRenderer extends Component {
             }));
     }
 
+    get displayOptionalFields() {
+        return this.getOptionalFields.length;
+    }
+
     nbRecordsInGroup(group) {
         if (group.isFolded) {
             return 0;

addons/web/static/src/views/list/list_renderer.xml

@@ -68,7 +68,7 @@
                         <td t-if="props.activeActions and props.activeActions.onDelete"/>
                     </tr>
                 </tfoot>
-                <Dropdown t-if="getOptionalFields.length" class="'o_optional_columns_dropdown'" position="'bottom-end'">
+                <Dropdown t-if="displayOptionalFields" class="'o_optional_columns_dropdown'" position="'bottom-end'">
                     <t t-set-slot="toggler">
                         <i class="oi oi-settings-adjust o_optional_columns_dropdown_toggle"/>
                     </t>