Use qdbus_cast

Let Qt do the boilerplat for us.

Does virtually the same under the hood (https://code.woboq.org/qt5/qtbase/src/dbus/qdbusargument.h.html#_Z10qdbus_castRK13QDBusArgumentPT_)
1 job for master in 9 minutes and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #59652
linux kf5-qt5 qt5.12

00:09:55

1.0%