LibnotifyNotification: add support for notification tag

We set the id of the notification if tag is provided.
As per the protocol, this should cause the old notification with same ID to be replaced.

brightray/browser/linux/libnotify_notification.cc

@@ -119,6 +119,11 @@ void LibnotifyNotification::Show(const base::string16& title,
     g_object_unref(pixbuf);
   }
 
+  if (!tag.empty()) {
+    GQuark id = g_quark_from_string(tag.c_str());
+    g_object_set(G_OBJECT(notification_), "id", id, NULL);
+  }
+
   GError* error = nullptr;
   libnotify_loader_.notify_notification_show(notification_, &error);
   if (error) {