Make it possible to pass a spa_node as the follower of the adapter. This
avoids us having to wrap it in a pw_impl_node in order to unwrap it
again for the adapter spa_node.
The proxy API is the one that we would like to expose for applications
and the other API is used internally when implementing modules or
factories.
The current pw_core object is really a context for all objects so
name it that way. It also makes it possible to rename pw_core_proxy
to pw_proxy later.
An adapter is like an audio-dsp node and client-stream combined.
It allows tighter control with the device (for rate control and
variable buffer sizes), software volume.
The idea is to also implement the client-stream with this
eventually.
