API Cleanup

Various changes to make for a nicer public API and better general tidiness, such as:

• ConfigSource and ContentSource interfaces with distinct convenience methods.
• Organize imports
• Use generics to avoid casting, where possible
• Introduce TreeDefinition base class so that ContentDefinition and ConfigDefinition can be distinct types (not one a subclass of the other)
• Don't use default methods on interfaces
• Remove interface methods that are too implementation-specific
• Use NodePath instead of String wherever possible
• etc....