feat(notifs): handle transient notifications properly

Transient notifications are not persistently shown in the dashboard, and
thus permanently expire after their timer runs out.

modules/notifications/Notification.qml

@@ -76,7 +76,7 @@ CustomRect {
             if (Math.abs(root.x) < Config.notifs.width * Config.notifs.clearThreshold)
                 root.x = 0;
             else if (root.inPopup)
-                root.notif.popup = null;
+                root.notif.clear();
             else
                 root.notif.notification.dismiss();
         }