Disable pre-heat button when not connected
Disable pre-heat button when not connected

This covers the case when there is no printer added as well as the case where a printer is added but not connected.

Contributes to issue CURA-3161.

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