When trying to exit the application, forcefully shutdown the backend exe.

Fixes CURA-1453 Cura in slicing loop (Arcus Error (6, native 54))

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -103,6 +103,9 @@ class CuraEngineBackend(Backend):
 
         return [Preferences.getInstance().getValue("backend/location"), "connect", "127.0.0.1:{0}".format(self._port), "-j", json_path, "-vv"]
 
+    def close(self):
+        self._terminate()   # Forcefully shutdown the backend.
+
     ##  Emitted when we get a message containing print duration and material amount. This also implies the slicing has finished.
     #   \param time The amount of time the print will take.
     #   \param material_amount The amount of material the print will use.