This class allows hosting several independent add-in engines in a single application domain. In general, applications use the AddinManager class to query and manage extensions. This class is static, so the API is easily accessible. However, some kind applications may need to use several isolated add-in engines, and in this case the AddinManager class can't be used, because it is bound to a single add-in engine. Those applications can instead create several instances of the AddinEngine class. Each add-in engine can be independently initialized with different add-in registries and extension models.