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


(cherry picked from commit 10e1755f)