manager: remove over-inclusion in header file

Decrease the manager.h file dependency map by removing
uneeded inclusion and small code refactoring to nicely decoupling
classes.

Change-Id: Ibde69e0a088e7e2fbd842e39301db5b3a4bc8c28
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>