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)

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.

This reverts commit daa49af5.