Deprecate download type

This patch removes the download type property from QtWebEngineCore and replaces
it with a simple boolean 'isSavePageDownload'. On the public API boundary the
type property is reimplemented via this boolean and documented as obsolete.

Rationale being that

  1. This feature seems to lack practical use cases, other than perhaps
     distinguishing save-page downloads from normal file downloads, which can be
     done in a much simpler way.

  2. This feature does not work as documented and never has. So far nobody has
     complained, hinting again at a lack of practical use cases.

  3. In order to fix it we would need to maintain patches on top of Chromium and
     Blink (we would, for example, need to propagate the DownloadAttribute type
     from Blink to Chromium to WebEngine).

[ChangeLog][Deprecation Notice] (QWebEngine)DownloadItem::type() is deprecated
and replaced with the newly introduced isSavePageDownload() property.

Task-number: QTBUG-62640
Change-Id: Icf4e1e5a635028986df7eab979f4c0527902ff0c
Reviewed-by: Michal Klocek <michal.klocek@qt.io>