allowed values are 'button' and 'toggle'
import BaseAction from "popups/BaseAction";
  new BaseAction("button")({
       id: "tweetablePopups.action.tweet",
       title: "Tweet Feature",
       className: "esri-icon-share2",
       image: "tweeter.png",
       isVisibleForFeature(feature){
           // only show feature if attribute exists
           return feature.attributes["attribute"] === true;
       },
       trigger(context){
             // do something
       }
    });
Creates a BaseAction clazz for extension.