Add and populate internal managerDevice property in managed device classes
I asked @degon what we could do to either prevent or at least better detect when supposedly managed devices were started externally outside of manager control. Apparently the framework has an assignment mode called Assignment.INTERNAL, which prevents a key from ever getting set by the GUI. This MR adds such a key called managerDevice, which is empty by default and filled by its own device ID when instantiated by the manager.
We can consider afterwards what to do with it, e.g. I was initially thinking of the manager having a list of such devices it finds in its domain.