This patch adds a QWebEngineDownloadItem class to the widget API
and extends QWebEngineProfile with a signal to notify about initiated
downloads, similar to the QML API.

[ChangeLog][QtWebEngineWidgets] Add widgets download API

Change-Id: I532de640934e1e93d63dfc347efb4113cb4a3d66
Reviewed-by: Jocelyn Turcotte <jturcotte@woboq.com>