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.