Disallow pre-heat temperature of 0 explicitly
Disallow pre-heat temperature of 0 explicitly

Because that signals to the printer that it should cancel pre-heating.

Contributes to issue CURA-3161.

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