* 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.
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
