Chainable Response in AppFramework

Makes is possible to chain method calls like in e.g. the routes (I'm aware the example makes no sense :smile: ):
```php
return $response->setEtag(md5('Hello world'))
	->setStatus(Http::STATUS_NOT_FOUND)
	->setLastModified(new DateTime('now'))
	->cacheFor(33)
	->addHeader('Cache-Control', 'must-revalidate');
```
@Raydiation @bartv2 @DeepDiver1975 @karlitschek @babelouest