Disallow pre-heat temperature of 0 explicitly

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

Contributes to issue CURA-3161.

resources/qml/PrintMonitor.qml

@@ -336,6 +336,10 @@ Column
                 {
                     return false; //Target temperature too high.
                 }
+                if (parseInt(preheatTemperatureInput.text) == 0)
+                {
+                    return false; //Setting the temperature to 0 is not allowed (since that cancels the pre-heating).
+                }
                 return true; //Preconditions are met.
             }
             anchors.right: parent.right