Fixes #1670. Add function that gets all assets added by extensions.

See #1670.
All extensions can use `getAssets` for a list of all assets added by extensions. E.g. in your extension, 

```
$assets = $this->getAssets();
$assets['js']; // all js files
$assets['css']; // all css files
\Dumper::dump($assets);
```

You can also access it via the global `{{ app.extensions.assets }}` in twig templates, which is a pretty nice addition.