The default value is a JSON containing 2 keys:
- the relative_url of an active process
- the list of active process image content to aggregate
By default, when the field is rendered for the first time, this tales expression is evaluated and a new active process is created at that point.
So, default json is like: {active_process: 'portal_activities/1234', image_list: []}
* store the active_process relative_url in the gadget state in the render method
* store the image_list in the gadget state in the render_method
* change getContent method to send a json build from the active_process and image_list state