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.

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.