temp: do not change preheat selections while in the numpad

it's possible to just use one selection list and unselect the rest,
but i think that there is some benefit in usability keeping the selections separated

panels/temperature.py

@@ -222,6 +222,7 @@ class TemperaturePanel(ScreenPanel):
             self.hide_numpad()
 
     def select_heater(self, widget, device):
+        if self.active_heater is None:
             if self.devices[device]["can_target"]:
                 if device in self.active_heaters:
                     self.active_heaters.pop(self.active_heaters.index(device))