This is a not *that* straight forward. For convenience, Devices
offers information from the DeviceClasses and used to get that stuff
from the engine. In order to solve this I've added a pointer to the
DeviceClass in Device. This however, implies that DeviceClasses
must be created before Devices and destroyed after them. As the
general concept of the system is built in such a way this isn't
an issue but it requires some extra throughts when tearing down
things.
