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 reverts commit daa49af5.

Exposing a spa_dict is necessary to allow using native pipewire API
that deals with these properties.

The internal structure change avoids mem copies when we need to
return a spa_dict.

This commits also removes exposing internal info structures via the
properties mechanism. This needs more thinking...

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

This allows plugins to provide custom interfaces on proxies

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

the GType cannot be in statically allocated data,
since it's not known at compile time

glib log domains are not like the gstreamer ones, where you can enable
many of them with a wildcard, therefore it is not particularly useful
to have different ones per file