device: Add a unique, persistent identifier
Currently there's no way to know across application restarts or even when restarting the device provider which device maps to one that was selected in a previous run. Some unique identifier for this would be useful for applications to allow users to select a device and then reliably get that very same device again at a future run.
While this can already be done per provider via the extra properties on the device this doesn't give us a common API for it yet. So I'd propose an additional property on GstDevice for this very purpose.
The main question then would be what we would do as a backward compatibility fallback.
CC @ocrete