6408: Convert Metadata Viewer from pyexiv2 to GExiv2.  Move Edit Exif Metadata gramplet into gramps-addons.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@21387 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
6408: Display GExiv2 version rather than pyexiv2 version

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@21389 4ae1f11a-8b86-4847-b8af-ab372f36d1fd