This allows calling code to add its own path validation without creating
a derived class.
Some notes on API decisions:
    - Since all current users of this functionality call the base class
      implementation in their derived function, no functionality is
      provided to completely replace the path validation function (as
      opposed to merely add checks). In the unlikely case that this is
      ever needed, we can easily add it.
    - The member function is called "setAdditionalValidator" rather than
      the shorter "addValidator" because the latter might suggest that
      repeated calls will chain the provided functions.
    - There is also no functionality to conveniently remove the
      additional validator, because such dynamic behavior was not needed
      so far.
This patch only does the minimum changes to the calling sites that are
required for them to continue compiling and working. Removal of the
derived classes that are no longer needed happens in a follow-up patch.

Change-Id: I5282835b5dd227149748a7f567006beb288d8aa3
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>