Don't evaluate default_value as fallback
Don't evaluate default_value as fallback

Just take the value itself without evaluating it. Otherwise enum values accidentally get evaluated as a variable.

Contributes to issue CURA-1758.

totalretribution/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,totalretribution/CuraEngine,alephobjects/CuraEngine,Ultimaker/CuraEngine,totalretribution/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine