Unified metadata container
Instead of maintaining one metadata container and all its supporting infrastructure for each type of metadata, there should be one unified metadata container for all metadata.
This will make it easier for applications to deal with metadata independent of its type. On the other hand, there should be supporting functions (filters) for cases where only a specific type of metadata is needed.
#585: Added new concrete and unified class Key. Currently called Key1 and not in use yet.
[unstable] #585: Replaced Xmpdatum with the new Tag(1) and a couple of ugly (temporary) hacks.