Add a way to know if QWebEngineUrlRequestInfo is a download

Downloads can happens in 2 ways:

1. The user navigate to a file that chromium cannot open (e.g. a zip file), or the HTTP headers contain an hint that the file should be downloaded
2. The user makes an explicit download request e.g. by using the DownloadImageToDisk action

In the first case, we cannot know if the request will lead to a download.

However in the second case, chromium knows that the request will lead to a download. This info should be exposed so that it is possible to take different actions in the interceptor when we know it is a download.

Example of use case: we want to prevent users from directly opening a video file, but let them download the file.