Generalize PrefService API

The PrefService was a refactoring of the Prefs class, which was bound to java.util.Preferences (see #102 for details). Thus the API is fairly specific to java.util.Preferences.

Areas to generalize:

• getters and setters for each type
• Maps that only hold String values
• Lists that only hold String values
• Remove the access to global, Class-less preferences. These only encourage name clashes for no concrete benefit.