Hide the slider controls when there are no layers
Hide the slider controls when there are no layers

(and code cleanup)

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