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...
