[Devel] Preferences and Persistence design 0.3.0

Hi all,

Please find attached version 0.3.0 of the Preferences and Persistence design, plus a document showing the changes between version 0.2.3 and 0.3.0.

This version includes more background research, some alternative designs for the system preferences application (§5.1.5) and clarifications of recommendations for storing secrets and passwords.

Is this one OK to upload to the wiki?

Philip