Promote WebEngineDownloadItem and WebEngine singleton to
public API and add DownloadView to the browser example.
This patch also adds profile support and webengine settings
to quicknanobrowser.

Change-Id: Ie81fb330e640fad7feec667a8af3afe67050693f
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>