After discussing things at the AGL May 2019 F2F meeting
and reflecting on the initial design of WirePlumber,
it became clear that it needed a fresh start.

This makes it more intuitive to get other attached interfaces
from the core

We are going to use this in other object types as well

* make the registries available as dynamic interfaces of the core
* improve a bit the C API of WpObject & WpInterfaceImpl