Update disallowed area when object's extruder is changed

This calls _updateDisallowedAreas() but not rebuild(), so the change is not visible at this moment.

Contributes to issue CURA-2625.

cura/BuildVolume.py

@@ -110,6 +110,9 @@ class BuildVolume(SceneNode):
                 per_mesh_stack = node.callDecoration("getStack")
                 if per_mesh_stack:
                     per_mesh_stack.propertyChanged.disconnect(self._onSettingPropertyChanged)
+                active_extruder_changed = node.callDecoration("getActiveExtruderChangedSignal")
+                if active_extruder_changed is not None:
+                    node.callDecoration("getActiveExtruderChangedSignal").disconnect(self._updateDisallowedAreas)
                 node.decoratorsChanged.disconnect(self._onNodeDecoratorChanged)
 
             self._scene_objects = new_scene_objects
@@ -122,6 +125,10 @@ class BuildVolume(SceneNode):
         per_mesh_stack = node.callDecoration("getStack")
         if per_mesh_stack:
             per_mesh_stack.propertyChanged.connect(self._onSettingPropertyChanged)
+        active_extruder_changed = node.callDecoration("getActiveExtruderChangedSignal")
+        if active_extruder_changed is not None:
+            active_extruder_changed.connect(self._updateDisallowedAreas)
+            self._updateDisallowedAreas()
 
     def setWidth(self, width):
         if width: self._width = width

cura/Settings/SettingOverrideDecorator.py

@@ -61,6 +61,12 @@ class SettingOverrideDecorator(SceneNodeDecorator):
     def getActiveExtruder(self):
         return self._extruder_stack
 
+    ##  Gets the signal that emits if the active extruder changed.
+    #
+    #   This can then be accessed via a decorator.
+    def getActiveExtruderChangedSignal(self):
+        return self.activeExtruderChanged
+
     ##  Gets the currently active extruders position
     #
     #   \return An extruder's position, or None if no position info is available.