Avoid another possible endless loop

This is a corner case, but it could, at least in theory, happen:
- WebEnginePart determines application/octet-stream as mimetype for a
  given URL and passes it to KonqMainWindow::openURL, which in turn uses
  UrlLoader
- UrlLoader uses OpenUrlJob to make another attempt at determine the
  mimetype and finds that it's actually a mimetype supported by
  WebEnginePart (for example, application/xml)
- if the user chose to embed the mimetype and the preferred service is
  WebEnginePart, we get an endless loop because WebEnginePart thinks it
  can't handle the URL (it finds again application/octet-stream), calls
  again KonqMainWindow::openURL and so on

To avoid this, when the preferred part is WebEnginePart but the URL has
already been processed by WebEnginePart, the second preferred part is
used. If there's only one availlable part, the user is asked wether to
open or save.