fix: handle multithreaded progress messages (CURA-781)
fix: handle multithreaded progress messages (CURA-781)

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