Fix a bug "Event loop closed" on xaal-shell

On Python 3.12 (new version), the watchdog_task raise a RuntimeError. To
avoid this, I need to cancel() all pending tasks

apps/tools/xaal/tools/toolbox.py

@@ -979,6 +979,17 @@ def shell():
         print("$ pip install ipython\n")
         exit(1)
 
+    # for a unknown reason, watchdog_task raise a RuntimeError: 
+    # "Event loop is closed" on the shell exit. To avoid issue
+    # we need to stop all tasks.
+    async def on_stop():
+        for task in eng.all_tasks():
+            logging.debug("Stopping task %s", task.get_name())
+            task.cancel()
+            await asyncio.sleep(0.1)
+
+    eng.on_stop(on_stop)
+
     logging.getLogger("parso").setLevel(logging.WARNING)
     logging.getLogger("blib2to3").setLevel(logging.WARNING)