Make extrudersChanged into a pyqtSignal
Make extrudersChanged into a pyqtSignal

This way we can listen for it on the cute side.

Contributes to issues CURA-340 and CURA-1278.

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