derive FileResourceBase virtually from PersistentObject

Preparation for multiple inheritance.
We need to adjust the PersistentPool load functions to avoid static_cast.

Change-Id: I95bb511b838ca2d1f2ed67386ae8ba821fad8d63
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Increase build graph version.

Should have been part of 1ce74f4c5e7d9e9ccb854c22fa8540ebcaded154.

Change-Id: I228fa7360d1b4b62941ffa9d8dd26b9b2e1187d2
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>