QWebEngineDownloadItem::type() is not working correctly

Its value is "DownloadAttribute" when an explicit download is initiated by QWebEnginePage::load(). We should add a new enum value for this case.

Furthermore, there are some strange things in the source code of this feature: https://codereview.qt-project.org/#/c/156951/

• Testing an enum value in DownloadManagerQt:

if (!isAttachment || !BrowserContextAdapterClient::UserRequested)

• m_downloadType is a member of DownloadManagerDelegateQt: It could cause problems in parallel downloads.

These need to be investigated.