avoids criticals and crashes when the core is destroyed earlier

+ add a method to retrieve the core
+ change the register function to not require the core as a parameter

Use proxy api in module-pipewire and simple-endpoint

See merge request gkiagia/wireplumber!8

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)

Proxy API

See merge request gkiagia/wireplumber!7

This can be used by code that depends on policy to re-evaluate
choices they have made in the past that could now change based
on the new policy.

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

The core may be the last object holding a reference to the endpoint,
so the unregister call may destroy it.

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())

Endpoint linking

See merge request gkiagia/wireplumber!5

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.

endpoint: fix bug when linking endpoints

See merge request gkiagia/wireplumber!4

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