Add a busy property to LayerView that indicates whether we are currently building top layers
Add a busy property to LayerView that indicates whether we are currently building top layers

Contributes to CURA-957

fieldOfView/Cura,senttech/Cura,ynotstartups/Wanhao,senttech/Cura,ynotstartups/Wanhao,Curahelper/Cura,hmflash/Cura,Curahelper/Cura,totalretribution/Cura,hmflash/Cura,totalretribution/Cura,fieldOfView/Cura