allow user to define new private tags from GUI

[irrer]

Sometimes users load files with undefined attributes that can not be viewed. A GUI interface would make it easy to do so.

The downside of this is yet more complexity, especially in keeping the list of known attributes in a file, keeping that file in a 'good' place, and not over-writing that file when a new version of the software is installed.

The workaround is to define them by editing the configuration file and restarting the application.