Add option to suppress duplicate notifications

Duplicate notification suppression is handy in case you run a chat client (like MS-Teams) in more than one instance on your machine (I always have two of them open). In such a case, each instance will send the same desktop notification, turning your machine into a "Whac-A-Mole" annoyance. Turning on the duplicate suppression will ignore any new notifications which have the same summary and body as any visible notifications.

The xfce4-notifyd-config tool has been amended to have a new toggle, to enable/disable this new feature. Default is "off", to stay backward compatible.

In the tests folder, there is a (manual) test script to see the result of the suppression.