Center of group node is now on correct position.
Center of group node is now on correct position.

Fixes Ultimaker/Cura#378

fxtentacle/Cura,senttech/Cura,Curahelper/Cura,senttech/Cura,fieldOfView/Cura,hmflash/Cura,ynotstartups/Wanhao,ynotstartups/Wanhao,bq/Ultimaker-Cura,totalretribution/Cura,fxtentacle/Cura,ad1217/Cura,hmflash/Cura,totalretribution/Cura,Curahelper/Cura,markwal/Cura,fieldOfView/Cura,bq/Ultimaker-Cura,ad1217/Cura,markwal/Cura