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

+ add a method to retrieve the core
+ change the register function to not require the core as a parameter

This is a cleaner way to interface with the remote pipewire daemon.
The WpRemote base class can be subclassed also for interfacing
with other daemons (hardware-specific managers, etc)

We need to delay the registration of the simple endpoint because
it needs to get some information from the "info" callback of pipewire
before we can use it in a link.

Only one endpoint of a certain media class can be selected at a time