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.
