Rename Shell Speed to Wall Speed
Rename Shell Speed to Wall Speed

It only applies to the walls, not the skin, after all.

Contributes to issue CURA-1183.

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