Fix empty dict as default arg

See e.g.
https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422

Author: eudoxos

cq_editor/widgets/object_tree.py

@@ -269,7 +269,9 @@ def addObjects(self,objects,clean=False,root=None):
             self.sigObjectsAdded[list].emit(ais_list)
 
     @pyqtSlot(object,str,object)
-    def addObject(self,obj,name='',options={}):
+    def addObject(self,obj,name='',options=None):
+
+        if options is None: options={}
 
         root = self.CQ