How to add traits dynamically based on the dropdown selection

**Dropdown**
![before](https://user-images.githubusercontent.com/19382262/58178404-c52c6a00-7cc3-11e9-8a70-e00e45ed71ee.jpg)

**Extra fields**
![after](https://user-images.githubusercontent.com/19382262/58178909-b5615580-7cc4-11e9-968e-d5b35781c39f.jpg)

Code is below, I have added the listener but not getting the option to add new fields to the current trait setting.

DomComps.addType('linking', {
        model: CustomType.model.extend({
            init() {
                this.listenTo(this, 'change:link-type', function() {
                    alert();
                });
            },
            defaults: Object.assign({}, CustomType.model.prototype.defaults, {
                traits: [{
                    label: 'Title',
                    name: 'title',
                    placeholder: 'Title'
                }, {
                    type: 'select',
                    label: 'Type',
                    name: 'link-type',
                    options: [
                        { value: 'link', name: 'Link' },
                        { value: 'popup', name: 'Popup' },
                    ],
                    changeProp: 1
                }, {
                    label: 'Link',
                    name: 'data-link',
                    placeholder: 'Link'
                }, {
                    label: 'Tracking',
                    name: 'data-track',
                    placeholder: 'Tracking Id'
                }],
            }),
        }),
        view: CustomType.view.extend({
            events: {
            },
        }),
    });