Remove "Save Page" web action from the standard context menu

The SavePage web action is only useful if the user code reacts on the
downloadRequested signal. Also, having SavePage available is not useful
for e.g. hybrid applications.

Extend the demobrowser example to show how to add the SavePage web action
to the context menu.

Change-Id: I979bcd9f5a25d2cae07ef819663afde17784440b
Task-number: QTBUG-52370
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>