Add iteration over extruder manager
Add iteration over extruder manager

This iteration iterates over the extruders.

Contributes to issues CURA-1278 and CURA-351.

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