add editable cols as a demo

src/client/js/components/PageEditor/GridEditModal.jsx

@@ -76,6 +76,9 @@ export default class GridEditModal extends React.PureComponent {
               </div>
               <div className="col-9">
                 <div className="row h-100 flex-nowrap overflow-auto">
+                  <div className="bg-dark grid-editable-col col-3"></div>
+                  <div className="bg-dark grid-editable-col col-5"></div>
+                  <div className="bg-dark grid-editable-col col-4"></div>
                 </div>
               </div>
             </div>