Restructure conversions for add. DSML converters

* Extract a base class for arbitrary conversion managers.
* Adjust the converter interface.
* Add a FixedPropertyLiteral extension to the DSML specific to the D3SE.
  It caches the values of referenced properties of model elements.
* Create a base evaluator that handles this literal.
* Add the conversion to the explorer init.

Signed-off-by: Alexander Diewald <diewald@fortiss.org>