- in Tool Milling made sure that deleting the only tool will not crash the application

- added a new parameter in Preferences to control the number of processes created by the Pool() - more processes better performance but also a lot of memory consumed
- made sure that the display of messages in the Status Bar is done asap

appPool.py

@@ -23,7 +23,7 @@ class WorkerPool(QtCore.QObject):
 
     def __init__(self):
         super(WorkerPool, self).__init__()
-        self.pool = Pool(cpu_count())
+        self.pool = Pool(int(cpu_count() / 4))
 
     def add_task(self, task):
         print("adding task", task)