preferences Storage importers/exporters. Password/encryption key
prompting is now down in centralized place. On import, the password
prompt is given if the file to be imported uses an importer
implementation that requires a password and no password is provided.
On export, the password prompt is given if the chosen version is for
an exporter that requires a password and no password was provided.
For instance, for automatic backups, a password could be stored in
preferences and provided to the exporter, so no password prompt would
be given.
