ui: fixed delay closing the GUI

When closing the GUI we were not stopping notifications channel in all
cases, causing some issues (delays, log in/out problems, ...).

Closes #1217

ui/bin/opensnitch-ui

@@ -243,3 +243,7 @@ Examples:
         on_exit()
     except Exception as e:
         print(e)
+    finally:
+        if service:
+            # finish gracefully, closing notifications channel.
+            service.close()

ui/opensnitch/service.py

@@ -743,6 +743,11 @@ class UIService(ui_pb2_grpc.UIServicer, QtWidgets.QGraphicsObject):
         elif kwargs['action'] == self.NODE_DELETE:
             self._delete_node(kwargs['peer'])
 
+    def OpenWindow(self):
+        self._stats_dialog.show()
+
+    def close(self):
+        self._on_close()
 
     def PostAlert(self, alert, context):
         proto, addr = self._get_peer(context.peer())
@@ -935,6 +940,3 @@ class UIService(ui_pb2_grpc.UIServicer, QtWidgets.QGraphicsObject):
                 context.cancel()
 
         return node_iter
-
-    def OpenWindow(self):
-        self._stats_dialog.show()