Since the alsa endpoints class was renamed...

When two clients have the same role, the current policy should
favor the newest client.

Error handling and fixes

See merge request gkiagia/wireplumber!21

Avoids crashing if a rescan is scheduled but the policy module is
destroyed in the meantime

* 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.

Delay unhandled clients

See merge request gkiagia/wireplumber!20

Fix doc warnings

See merge request gkiagia/wireplumber!19

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