this allows to put the proxy definitions alongside with the
actually implementing service, which makes much more sense
than having all in one huge interfaceproxy.cpp file
DisplayService is intended to implement the Display facade and additionally
manage N display output sinks (DisplayerSlot). Now the playback-controller
contains just the bare controller logic, all services like tick generation
or signal dispatching have been factored out... makes me feel better :-)
this allows the C++ version to provide automatic lifecycle management for
the play process, while both versions of the API (C and C++) impose only
one level of indirection.
