Force recreating the socket when using external-backend

This may recreate the socket with a new port number but at least it
recreates the socket now.

Partially fixes CURA-1877

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -181,6 +181,11 @@ class CuraEngineBackend(Backend):
         self.slicingCancelled.emit()
         self.processingProgress.emit(0)
         Logger.log("d", "Attempting to kill the engine process")
+
+        if Application.getInstance().getCommandLineOption("external-backend", False):
+            self._createSocket()
+            return
+
         if self._process is not None:
             Logger.log("d", "Killing engine process")
             try: