This will allow us to generate code that handle and provide easy access
to metadata stored in TIFF's tags. The generator is a Python script, and
it output both TIFFMetadata.h and TIFFTagHandler.cpp files.
The generator will definitely need some update to support all TIFF and
EXIF tags, but that will still be easier than writing everything
ourselves.
Some small modifications are needed in TIFFLoader.cpp to make it
compatible with the new `Metadata` class.
