Metadata category manipulation

A metadata object contains a number of categories, each containing multiple component which store the actual metadata. Here are the routines for manipulation the categories.

See NMDObjectTryGetCategory(), NMDObjectGetCategory(), NMDAllocateCategory(), NMDObjectAllocateNewCategory(), NMDObjectGetOrCreateCategory(), NMDRemoveCategory(), NMDGetCategories(), NMDCopyCategory().