Remove binding in Stateful#observe() and Destroyable#own()

[wkeese]

Stateful#observe() and Destroyable#own() automatically bind the callback to this... which was a nice feature until ES6 came along.

Now, code like this has unexpected results:

const MyClass = dcl (null, {
    myWidget: ...,

    myMethod: function () {
         this.myWidget.observe(props => console.log(this));
    }
}
```

In the above code it's natural to assume that the `console.log(this)` will print`MyClass`, but it actually refers to `myWidget`,