save the application name in the Exif.Image.ProcessingSoftware tag

the Exif.Image.Software contains the name and version of the firmware of
the camera as described here: http://www.exiv2.org/tags.html

extensions/exiv2_tools/exiv2-utils.cpp

@@ -959,7 +959,7 @@ exiv2_write_metadata_private (Exiv2::Image::AutoPtr  image,
 	if (g_file_info_get_attribute_boolean (info, "gth::file::image-changed")) {
 		if (software_name == NULL)
 			software_name = g_strconcat (g_get_application_name (), " ", VERSION, NULL);
-		ed["Exif.Image.Software"] = software_name;
+		ed["Exif.Image.ProcessingSoftware"] = software_name;
 	}
 
 	// Update the dimension tags with actual image values