This allows linking the client to the stream that has the same name
as the role that it requests.

It so happens that sometimes the client proxy is created and destroyed
immediately, almost instantly, which causes this code to crash in case
the proxy pointer is gone when our idle callback tries to destroy it.

This change makes the whole operation safe.

Currently, the stream names have to be duplicated in the configuration
file. This is going to change in a future version.

... and simplify some code there

This is currently the documented way of describing controls
that have an effect on all streams

This is a bit of a hack, temporarily needed for AGL.

Currently this will just grant full access to all clients.
A future version will refine that to grant access to specific
objects only.

This is a situation that always happens on startup, so let's not
spam with warnings

spa audioconvert (the backend of audio-dsp) does not adevertise
property changes, therefore subscribe_params() is useless.
In order to get the real value from the node we need to call
enum_params() manually.

https://github.com/PipeWire/pipewire/issues/157

This provides high level volume controls for the AGL audiomixer
binding and the applications using it.

We now have always a "selected" endpoint for each direction (source, sink)
that is picked as the first available endpoint at startup,
and it changes automatically if the currently selected endpoint is removed

This avoids the theoretical crash that would happen if you change
the control value on an endpoint whose client-endpoint proxy has
already been destroyed