adds hooks for setting page layout and shell

Need to figure out where this code should live:

```
	$params = array();
	$params['segments'] = _elgg_services()->request->getUrlSegments();
	if ($params['segments']) {
		$params['identifier'] = array_shift($params['segments']);
	} else {
		$params['identifier'] = '';
	}
```

Should the request provide it? It is also in the Router.