- hided the main UI on application quit to create a user experience of a shutdown without lag

- added a way to terminate QThreads safely by waiting; should be much safer
- made sure that the ArgsThread class receive the signal to stop
- made sure that on application shutdown, all workers will quit before the actual exit

appPlugins/ToolPDF.py

@@ -378,7 +378,7 @@ class ToolPDF(AppTool):
             pass
 
         self.check_thread.timeout.connect(self.periodic_check_handler)
-        self.check_thread.start(QtCore.QThread.Priority.HighPriority)
+        self.check_thread.start(QtCore.QThread.Priority.HighPriority)   # noqa
 
     def periodic_check_handler(self):
         """