Polish the HTTP example.

- Remove unneeded member variables.
- Use new connection syntax.
- Streamline code.
- Add a QCheckBox for launching the file after download
  and make the default file name and download directory configureable.
- Make status messages more verbose.
- Set Password echo mode on authentication dialog.
- Extract the progress dialog to a separate class
  that is directly connected to the QNetworkReply, which
  is created on demand. Set set minimum  and duration on it.
  This fixes a crash that currently occurs when clicking "Abort"
  on the SSL error dialog and "Cancel" on the progress dialog that is
  then re-shown due to its internal force timer/minimum duration handling.
- Resize according to screen size.

Task-number: QTBUG-48332
Change-Id: Ia2611e63fe96d6f49e4cdd06049a206ddb2c2864
Reviewed-by: David Faure <david.faure@kdab.com>