Comment 0 for bug 1424023

In the helper I need to remove all messages related to given tag before I post a new notification with same tag. Otherwise I would flood the user with notification. If this could be indicated in the helper output, I would not have to use DBus to clear notifications and add a bunch of asynchronous code specifically for that reason. replace_tag is only available on the server, should also be a local feature on the client side.