Convert PrefService to Handler API

[hinerm]

After #102 is complete, with the conversion from a static utility class to a service, we have unfortunately lost some power to adapt behavior to individual data types - either the whole service is superceded, or it's not.

Thus it would be nice to refactor this service to use the Handler pattern, such that there would just be a few base put/get methods that delegated to appropriate handlers. Then the handlers of a single type could be provided and overridden.

[ctrueden]

I think it is OK to impose a single preference storage mechanism for the whole application. Overriding the service to control that in a different way is still possible. Someone could create their own service with handler plugins if they want. I don't know a concrete use case for having it work this way in core.