Completely rewrite metadata handling using gexiv2

Based on original patches from Hartmut Kuhse and modified
by Michael Natterer. Changes include:

- remove libexif dependency and add a hard dependency on gexiv2
- typedef GExiv2Metadata to GimpMetadata to avoid having to
  include gexiv2 globally
- add basic GimpMetadata handling functions to libgimpbase
- add image and image file specific metadata functions to libgimp,
  including the exif orientation image rotate dialog
- port plug-ins to use the new APIs
- port file-tiff-save's UI to GtkBuilder
- add new plug-in "metadata" to view the image's metadata
- keep metadata around as GimpImage member in the core
- update the image's metadata on image size, resolution and precision
  changes
- obsolete the old metadata parasites
- migrate the old parasites to new GimpMetadata object on XCF load