Rename SeContainerInitializer.addInterceptors()/addDecorators()/addAlternatives()/addAlternativeStereotypes() and clarify the purpose

These methods should not add any class to the set of bean classes for the synthetic bean archive. The purpose is solely to compensate the absence of the beans.xml descriptor.

The propsal is to rename:

• addInterceptors() -> enableInterceptors()
• addDecorators() -> enableDecorators()
• addAlternatives() -> selectAlternatives()
• addAlternativeStereotypes() -> selectAlternativeStereotypes()

The new names correspond to the spec wording (see for example 5.1.1. Declaring selected alternatives).