js: don't create invalid menus by default

When a new "Menu" type de champ is added, it comes pre-filled with a
menu title – and nothing else. Which is confusing, and invalid.

Instead pre-fill the type de champ with actual values (no titles).

app/javascript/components/TypesDeChampEditor/typeDeChampsReducer.js

@@ -110,7 +110,7 @@ function updateTypeDeChamp(
         break;
       case 'drop_down_list':
       case 'multiple_drop_down_list':
-        typeDeChamp.drop_down_list_value = '--Premier élément du menu--\n';
+        typeDeChamp.drop_down_list_value = 'Premier choix\nDeuxième choix';
     }
   }