This is a cleaner way to interface with the remote pipewire daemon.
The WpRemote base class can be subclassed also for interfacing
with other daemons (hardware-specific managers, etc)

We need to delay the registration of the simple endpoint because
it needs to get some information from the "info" callback of pipewire
before we can use it in a link.

Only one endpoint of a certain media class can be selected at a time

the stream id is optional, a control may apply globally to all streams

This shall be a temporary method of selecting which endpoint
is going to be the default input or output, for testing purposes

- Set the node proxy to NULL when it is destroyed externally
so that we don't try to destroy it a second time in finalize.
- Don't remove the dsp_listener, as this is already done inside
the node proxy destroy sequence (but keep removing the node
listener to avoid calling node_proxy_destroy() from inside
finalize())

This is implemented in a slightly hacky way, we register the GMainLoop
as a global object and use it from the module to quit the daemon.
This is bad design because the module assumes it is loaded inside
our daemon.

In the future, this should change. It looks like we should have an
object that tracks the state of PipeWire and main() should track
state changes of that object and decide what to do.

This allows loading of config and modules when deployed in
the target system without setting environment variables.

SPA_POD_Id is reserved for spa type IDs, not arbitrary implementation IDs

This changes the registration point to be the endpoint and not the
session manager, as it seems easier to implement (the implementation
will not need to keep a pointer to the session manager around)

* Make streams a GVariant array, for future-proofness
* Add API for controls (volume, mute, brightness, contrast, etc...)
* Remove API for profiles (it's not well-thought; may be re-added
  in the future)

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.