[REF] web_editor: cancel method of RTE should be async

and return its deferred. This commit makes the behavior consistent with
the `save` method.

addons/web_editor/static/src/js/editor.js

@@ -130,7 +130,7 @@ editor.Class = Widget.extend({
         return this.rte.save();
     },
     cancel: function () {
-        new $.Deferred(function (d) {
+        return new $.Deferred(function (d) {
             if (!rte.history.getEditableHasUndo().length) {
                 return d.resolve();
             }