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.

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

and also implement
* global-added/removed signals
* a foreach function to iterate through globals
* registering modules and factories with the same key

g_weak_ref_clear destroys the weak ref, so it will crash
when _finalize() tries to destroy it again

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)