Support configurable PrefStores

[ctrueden]

The PrefService is currently backed by the Java Preferences API always. There is a separate node for the global settings, and per class, but it is not configurable.

KNIME would like to define its own PrefStore (or perhaps give a PrefStoreFactory which generates them) which would be a new interface offering all the current getters and setters of PrefService. And then PrefService itself would basically become just a map, with PrefStore get(String key) and void put(String key, PrefStore store). Or we could go with PrefStore store(String key, PrefStoreFactory fac) or something.

If we time this redesign properly, it can be included with SJC3.

/cc @dietzc