add an event for things like React components to listen for webclient updates

[Pomax]

e.g.

const userInterface = ClientServer.generateClientServer(WebClientClass)
const { Component } = require('react');

const MyInterface extends Component {
  constructor(props) {
    super(props);
    userInterface.addEventListner("webclient:update", evt => {
      this.setState(evt.detail.update);
    });
  }
  ...
}

[Pomax]

added