ui,stats: simplified columns configuration

There're only two points where we need to reconfigure the columns:
on showEvent() and when saving the settings.

There should be a slight performance improvement.

ui/opensnitch/dialogs/stats.py

@@ -656,6 +656,7 @@ def showEvent(self, event):
         self._add_rulesTree_fw_chains()
         self.setWindowTitle(window_title)
         self._refresh_active_table()
+        self._show_columns()
 
     def eventFilter(self, source, event):
         if event.type() == QtCore.QEvent.KeyPress:
@@ -2786,5 +2787,3 @@ def setQuery(self, model, q):
                     self.labelRowsCount.setText("")
             except Exception as e:
                 print(self._address, "setQuery() exception: ", e)
-            finally:
-                self._show_columns()