Fix for #8542 - MMU - Allow user to choose initial extruder but then also change extruder at layer height

1. Fix for detection of the DoubleSlicer mode => An extruder for the Height Range modifiers have to be ignored, if it's default.
2. OSX only: Extruder selection wasn't editable for Height Range modifiers

src/slic3r/GUI/GUI_Preview.cpp

@@ -804,6 +804,7 @@ void Preview::update_layers_slider_mode()
 
                     for (const auto& range : object->layer_config_ranges)
                         if (range.second.has("extruder") &&
+                            range.second.option("extruder")->getInt() != 0 && // extruder isn't default
                             range.second.option("extruder")->getInt() != extruder)
                             return false;
                 }