Revert "Simplify Widget class constructor"

[ibgreen]

Reverts #9815

After additional thought, I am not comfortable with making "negative changes" to our API based on how one current LLM model performs on it.

Basically, having an API that implicitly expects a static member be defined for subclassing to work is what I consider surprising to end users and such "magic" is not something I would like to find in any API I am using.

I expect that we will need to follow up with discussion in slack. Perhaps someone actually prefers this design and then that is of course a different discussion.

[coveralls]

coverage: 91.144%. remained the same
when pulling c033a11 on revert-9815-x/widget-class-2
into 5da7942 on master.

[Pessimistress]

Our standard code pattern (and in my opinion easier to read) is to merge user props with default in each subclass constructor, like so:

class MyWidget extends Widget {
  constructor(props) {
    super({
      optionA: true,
      optionB: 1,
      ...props
    });
  }
}

The static defaultProps field is used by Layer classes for chained inheritance, which is not needed in the widget system.

[ibgreen]

Our standard code pattern (and in my opinion easier to read) is to merge user props with default in each subclass constructor, like so:

Yes, perhaps that is all we need. Compact and explicit.

But can we make this fully typed? And ensure we have all the required props set up? The given example omits the typing...

class Widget<WidgetPropsT> {
  props: Required<WidgetPropsT>;
  constructor(props: Required<WidgetPropsT>) { // do we need to omit WidgetProps here?
    ...
  }
}

class MyWidget extends Widget<MyWidgetProps> {
  constructor(props: MyWidgetProps) {
    super({
      optionA: true,
      optionB: 1,
      ...props
    });
  }
}

---

Context: The thing that made me most uncomfortable is that we are going around the type system, and rely on a "magic" static property to be declared, i.e. the code below.

 constructor(props: PropsT) {
    this.props = {
      // @ts-expect-error `defaultProps` may not exist on constructor
      ...(this.constructor.defaultProps as Required<PropsT>),
      ...props
    };