first draft for making the encryption layer independent

first draft for making the encryption layer independent

WIP.

In Deltatar, we cannot use the ctor itself to set up the
encryption because it is neutral wrt. reading / writing.
Only once one of the entry points:

- ``.list_backup()``,
- ``.create_full_backup()``, ``.create_diff_backup()``, or
- ``.restore_backup()``.

are invoked do we know what the object’s intended use is.
Thus we hook the encryption handler somewhere in there.