Constants (as generated by ApplicationModule::constant()) were causing a
logic_error if accessors of different types were using the same constant
(created with the same value and the same value type upon creation).
This is possible since the type passed to constant() does not have to be
the same as the type of the accessor.

This is now fixed by giving each constant a fully unique name, so every
accessor using a constant will be in its own little variable network.

This happens if the control system adapter optimises out the PV.

A PV which is received by a module and also used as a trigger for a
DeviceModule was "deleted" from the model, if the module is unregistered
from its owner. This deletion is now prevented by adding a trigger
relationship to the model (which was anyway missing).

Note: the "deletion" never was a real deletion, instead only all
relationships are removed. As a consequence, the DeviceModule was still
pointing to the "deleted" vertex resulting in a wrong connection later.