Add convenience method to save page

5.7 added new action, QWebEnginePage::SavePage and corresponding methods in QWebEngineDownloadItem.
But this workflow is not intuitive and not documented enough.

More intuitive approach could be adding some method like:

void savePage(const QString &path, SavePageFormat format)

Both arguments could be optional, path can be still set on QWebEngineDownloadItem and format can still default to something sane.