Rework wallet file implementation to embed permissions

[ValentinTrinque]

I tried to implement the permissions system without touching to the current implementation of the wallet, to lower the amount of work. But, now, I realise that the permissions file and the wallet file can get desynchronised in ways I really didn’t anticipated (example: keys specified in the permissions file might have been removed from the wallet file, the wallet file could also be renamed, a different wallet could have the same name as the one in the permissions file but with different keys, etc).

Trying to handle these problems with my current implementation (2 separate files) creates all kind of UX problem and fragile error handling.

As a result, I need to embed the permissions inside the wallet file. This way I can enforce strong consistency between permissions and keys, and bypass all wallet rename problem. Also, it makes the backup of the wallet and its associated permissions much easier.