avoids criticals and crashes when the core is destroyed earlier

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.

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