Add QML download API

This patch exposes downloadStarted and downloadFinished
signals on the WebEngineProfile to notify about downloads.
The WebEngineDownloadItem exposes a subset of Chromium's
content::DownloadItem functionality.
For now we expose minimal requirements to be able to control
downloads in QML but this can be extended in the future.
This patch also adds a DownloadView to quicktestbrowser
to demonstrate the usage of the new API.

[ChangeLog][QtWebEngineQML] Add QtQuick download API

Change-Id: I8d8f0daf02c4e0151000427fc2a4b37d28b9db52
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>