message: Check validity of the DBus interface instead of signals (linux)
  Closes
diff: @@ -21,9 +21,10 @@ SystemTray::SystemTray(MainWindowController *parent, QIcon defaultIcon) :
  this, SLOT(displayReminder(QString,QString))); // NOLINT
  notifications = new QDBusInterface("org.freedesktop.Notifications", "/org/freedesktop/Notifications", "org.freedesktop.Notifications", QDBusConnection::sessionBus(), this);
  + notificationsPresent = notifications->isValid();
  - notificationsPresent = notificationsPresent && connect(notifications, SIGNAL(NotificationClosed(uint,uint)), this, SLOT(notificationClosed(uint,uint)));
  - notificationsPresent = notificationsPresent && connect(notifications, SIGNAL(ActionInvoked(uint,QString)), this, SLOT(notificationActionInvoked(uint,QString)));
  + connect(notifications, SIGNAL(NotificationClosed(uint,uint)), this, SLOT(notificationClosed(uint,uint)));
  + connect(notifications, SIGNAL(ActionInvoked(uint,QString)), this, SLOT(notificationActionInvoked(uint,QString)));
  auto pendingCall = notifications->asyncCall("GetCapabilities");
  auto watcher = new QDBusPendingCallWatcher(pendingCall, this);
  