Refactoring: The preferences system

• IMP's original preferences system wasn't terrible – there was an API that could be implemented for conceivably pretty much anything.
• However, it needed a get and set function for each preference, and we needed to add a lot more preferences for 2.3.
• Also, other Horde applications needed preferences.
• So the system was rewritten to be a general Horde service.
• Now it's extremely flexible, allows for sharing of preferences between applications, builds the user interface dynamically, etc.