A bundle is a collection of resources and code that can be used to
enhance an existing application or tool dynamically using the NSBundle
class from the GNUstep base library.
— Bundles: BUNDLE_NAME
The name of your bundle, without the suffix (.bundle).
— Bundles: PRINCIPAL_CLASS
xxx_PRINCIPAL_CLASS is the name of the principal class in
your bundle. This is the class that will be loaded first by GNUstep
when your bundle is loaded. Hence, it is best to put your +initialize method in here.