Add watch decorator that will invalidate when the property is set

[agubler]

Enhancement

Often widgets contain internal state that, when set, require the user to call this.invalidate() to ensure the widget is re-rendered.

Add a decorator for watching internal properties so that when they are set it will automatically call invalidate

class MyWidget extends WidgetBase {

    @watch()
    private _myState = 0;

    private _increment() {
         this._myState = this._myState + 1; // this automatically calls invalidate
    }

    protected render() {
        return v('div', [ 
            v('button', { onclick: this._increment }, [ 'Increment' ]),
            `${this._myState}`)
        ]);
    }
}