manager: decouple interface from implementation

This patch uses a pimpl idiom to decouple
Manager class interface (public API) from its implementation
(private members).
A back-pointer is also used by private implementation.
It's a first step to reduce the complexity and dependency
of this fat class. But this patch tries to keep the task atomic.

No behavioral changes! But the public API has been
slightly modified to arrange the pimpl code.

Change-Id: I01f15bab693397bc4c5a0f73393906110008488f
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>