ProgressManager: Hide some internally used function from .h interface.

The indirection of the "virtual + implementation inheritance" pattern is
kind of subset of the "static" pattern, no need to have both.

Change-Id: I4e2c2742dae94819ee05c7925c02b954a8c6294b
Reviewed-by: Eike Ziller <eike.ziller@digia.com>