`this` in event handler of `nw.Window` should point to the nw.Window object

To be backward compatible with nw 0.12, `this` in event handler of `nw.Window` should point to the nw.Window object.

``` javascript
var win = nw.Window.get();
win.on('resize', function() {
    console.log('win === this ? ' + (win === this));
});
win.resizeTo(100, 100);
```
