refactor: moved GcodeLayerThreader implementation to below class definition (CURA-3339)
refactor: moved GcodeLayerThreader implementation to below class definition (CURA-3339)

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