Fix: after extruders switch do not trigger slicing, provided a new signal

which is triggered after adding extruder and it subscribers for property changes

cura/Settings/ExtruderManager.py

@@ -49,6 +49,9 @@ class ExtruderManager(QObject):
     ##  Notify when the user switches the currently active extruder.
     activeExtruderChanged = pyqtSignal()
 
+    ## The signal notifies subscribers if extruders are added
+    extrudersAdded = pyqtSignal()
+
     ##  Gets the unique identifier of the currently active extruder stack.
     #
     #   The currently active extruder stack is the stack that is currently being
@@ -406,6 +409,7 @@ class ExtruderManager(QObject):
 
             if extruders_changed:
                 self.extrudersChanged.emit(global_stack_id)
+                self.extrudersAdded.emit()
                 self.setActiveExtruderIndex(0)
 
     ##  Get all extruder values for a certain setting.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -88,7 +88,7 @@ class CuraEngineBackend(QObject, Backend):
         #
         self._global_container_stack = None
         Application.getInstance().globalContainerStackChanged.connect(self._onGlobalStackChanged)
-        Application.getInstance().getExtruderManager().activeExtruderChanged.connect(self._onGlobalStackChanged)
+        Application.getInstance().getExtruderManager().extrudersAdded.connect(self._onGlobalStackChanged)
         self._onGlobalStackChanged()
 
         Application.getInstance().stacksValidationFinished.connect(self._onStackErrorCheckFinished)