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)

* 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)