Callbacks for Web Components

[marschall]

This is a brainstorming issue for further Web Components improvements aiming at better integration with other components and forms. First and foremost attributeChangedCallback() since attribute values are the interface a Web Component presents to the rest of the page. Here are some of the questions that we need to think about

• observedAttributes must be static, how can we make this depending on the Seaside component class?
• What should be interface for listening look like? An overridden method? A block?
• How should a component be able to respond? Always a full component render?
• Should we add an API to set attribute values on other components

Ideally this could then be further extended to support custom form elements using ElementInternals and checkValidity(), reportValidity(), setFormValue(), setValidity() methods.