Fix potential concurrency of finishing before connected to finishing

This was a concurrency issue: If the slicing was very fast, it could finish slicing before the listener was connected to the message of being finished. Therefore, we should connect to being finished before we even start the start-slice job.

Contributes to issue CURA-1278.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -155,8 +155,8 @@ class CuraEngineBackend(Backend):
         self.slicingStarted.emit()
 
         job = StartSliceJob.StartSliceJob(self._socket)
-        job.start()
         job.finished.connect(self._onStartSliceCompleted)
+        job.start()
 
     def _terminate(self):
         self._slicing = False