QDBusTrayIcon does not support 2 QSystemTrayIcon

When the QDBusTrayIcon (default platform theme, not the KDE one) is active, you can only have on QSystemTrayIcon.

This code does not show any QSystemTrayIcon:

#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QImage img(64,64, QImage::Format_ARGB32);
    img.fill(Qt::red);
    QSystemTrayIcon sti(QIcon(QPixmap::fromImage(img)));
    sti.show();

    img.fill(Qt::blue);
    QSystemTrayIcon sti2(QIcon(QPixmap::fromImage(img)));
    sti2.show();

    return a.exec();
}

Warning message:

```
QDBusMenuConnection::registerTrayIcon: failed to register "org.kde.StatusNotifierItem-20332-2" "/StatusNotifierItem
```

The reason is simple: connection().registerObject(StatusNotifierItemPath, item); tries to register twice the same path! and when this fails, it is unregistered from QDBusMenuConnection::unregisterTrayIcon which unregister the peviously registered QSystemTrayIcon