Prevent the backend attempting to reslice everytime a preference changed

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -229,6 +229,7 @@ class CuraEngineBackend(QObject, Backend):
         if not self._build_plates_to_be_sliced:
             self.processingProgress.emit(1.0)
             Logger.log("w", "Slice unnecessary, nothing has changed that needs reslicing.")
+            self.setState(BackendState.Done)
             return
 
         if self._process_layers_job:

resources/qml/ActionPanel/SliceProcessWidget.qml

@@ -134,10 +134,14 @@ Column
         onPreferenceChanged:
         {
             var autoSlice = UM.Preferences.getValue("general/auto_slice")
-            prepareButtons.autoSlice = autoSlice
-            if(autoSlice)
+            print(prepareButtons.autoSlice, autoSlice)
+            if(prepareButtons.autoSlice != autoSlice)
             {
-                CuraApplication.backend.forceSlice()
+                prepareButtons.autoSlice = autoSlice
+                if(autoSlice)
+                {
+                    CuraApplication.backend.forceSlice()
+                }
             }
         }
     }