* Every client has a priority based on its role
* For playback, we allow only a single client to play at a time
* For capture, we allow all clients to capture simultaneously
* Every time the "selected" device changes (either because devices
  are discovered/removed or because the user changed the selection),
  the clients are re-linked to the new "selected" device.
* When a playback client quits and there are others waiting unlinked,
  the highest priority one is linked automatically.
* This also properly fixes re-linking the correct client(s) to the
  correct device(s) when wireplumber exits and restarts.

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

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

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