Extended ConfiguratorElement and ResourceViewers

All ResourceViewers must inherit ResourceViewer as follows:

```javascript
sewi.AResourceViewer = function(options) {
...
}
sewi.inherits(sewi.AResourceViewer, sewi.ResourceViewer);

sewi.AResourceViewer.prototype.aFunction = function() {
...
```

ResourceViewer now has 2 built-in events:

- `'Closing'` - Close button is clicked
- `'FullscreenToggled'` - Fullscreen button is clicked

All ConfiguratorElements (including ResourceViewers) can now use `trigger()` and `on()` directly, instead of using `getDOM()` as a proxy.