cascading keyring

Consider the following scenario:
- software X v1 comes out, using `keyring` as a backend to store credential.
- someone use it on a system where only the fallback encrypted file backend is supported, credentials are stored there.
- software X v2 comes out, with an updated `keyring` that support a real keyring for this system
- user software X, the new backend is selected and all the credentials are not available anymore :-(

What about a "cascading keyring" that would wrap all the available backend for the system at hand and cascade the reads to the next backend if the previous one doesn't have it ? That would at least fix the disappearing credentials.

Bonus point would be to have a way to move all the credentials in the same backend, either up or down the stack to consolidate the storage in a single backend. That would allow to implement automatic migrations.