Add a button to send test notifications from

extension preferences widget.

Since in the preferences run in a separate process which
doesn't have access to main UI process,
it is not possible to send notification using Main.notify.
Hence, we use D-Bus to send test notification.

metadata.json

@@ -1,6 +1,6 @@
 {
   "name": "Notification Banner Reloaded",
-  "version": 8,
+  "version": 9,
   "_generated": "Generated by SweetTooth, do not edit",
   "description": "Configure notification banner position and animation to your liking",
   "uuid": "notification-banner-reloaded@marcinjakubowski.github.com",