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What is this?

This is the beginning of an editor for metadata.  It is far from
complete yet.

Current status
- It does not read metadata, except if there is an XMP block in the
  image file.
- It does not write metadata, it can only export XMP into a separate
- It does not edit metadata, except for some very limited editing of
  basic scalar types (strings, integers, ...).  It is currently not
  possible to add new properties using the editor (but this can be
  done via the PDB call plug_in_metadata_set_simple()).
- Basically, the current code is only useful for viewing some XMP
  metadata if the image file contains an XMP block.  The import/export
  functions are also supposed to work.  Import merges, so it is
  possible for example to import a license file such as the XMP files
  from Creative Commons in order to apply a license to an image.
- The metadata is saved in a persistent parasite that is overwritten
  after each call.  Caution: the editor should not be open while you
  are trying to test some of the PDB calls, otherwise you would
  discard all changes as soon as you press OK.  This is a problem that
  cannot be solved easily as long as the metadata editor is a plug-in.

How to find test images?

The easiest way is to try a Google search for "<?xpacket".  This is
the marker that is used at the beginning of XMP packets.  Many images
or PDF files are incorrectly served as text/plain and therefore
indexed by Google as text files.  By looking for XMP tags such as
"<?xpacket", it is possible to find a large number of files containing
XMP metadata.

Work in progress:
- improve XMP code generation (xmp-gen.c) - currently, some structured
  property types are not written back to the buffer, so the round-trip
  decode/encode can silently discard some properties
- add tabs for easy editing of metadata, connect them to the tree
- fix the conversion to/from EXIF (exif-parse.c, exif-gen.c)
- register PDB functions correctly (for setting/getting metadata)
- update the JPEG plug-in

Near future:
- add custom callbacks for editing in the tree, validate entries, ...
- smarter display of rational values, closed sets, ...
- improve the layout of the various tabs

Not-so-near future:
- move some of this stuff into the core

Files modified (2004-08-24):                    (added /plug-ins/metadata/Makefile)
  menus/         (added placeholder "<Image>/File/Info")
  plug-ins/            (added "metadata" in SUBDIRS)
  plug-ins/common/jpeg.c          (changed to get/set metadata through PDB)
New files:
  plug-ins/metadata/   (generates to get a Makefile)
  plug-ins/metadata/metadata.c    (main part: registers the plug-in, etc.)
  plug-ins/metadata/interface.c   (user interface: widgets and callbacks)
  plug-ins/metadata/xmp-model.c   (model for the treeview, list of XMP schemas)
  plug-ins/metadata/xmp-parse.c   (simple parser for XMP metadata)
  plug-ins/metadata/xmp-gen.c     (generates XMP metadata from the tree model)
  plug-ins/metadata/exif-parse.c  (converts EXIF into the XMP tree model)
  plug-ins/metadata/exif-gen.c    (generates EXIF metadata from the tree model)