WIP: Port away from Qt Webkit to Qt WebEngine

Because Qt Webkit has been deprecated for long. There are still
a few todos marked in here, but they do require quite some refactoring
and this commit is already big.

Current known defects:
* Downloading images served over http/https fails. A messagebox is shown
  with an error. In current master it fails silently on my system.
* The replacement for ErrorCheckingPage does not work.
* There are some encoding problem with html mails.
* UrlSchemeHandler::requestStarted is this called in it's own thread?
  Does it need locking of some kind?
* The unit test "test_Html_formatting" fails. In current master it
  passes on my system.

BUG: 365299