fix: don't zero extruder value after toolchange (CURA-959)
fix: don't zero extruder value after toolchange (CURA-959)

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