Improve the saving abilities of WebEnginePart
In particular:
- Add a "Save As" menu entry which allows to save the current page (useful in particular when WebEnginePart displays images, text files and so on as HTML pages)
- Add a "Save Full Page As" menu entry which saves the page and the external resources it needs (as described in QWebEngineDownloadItem::SavePageFormat). This replaces the non-working "Save Frame" menu entry
- Make the "Save Media" context menu entry actually work