uiutils: Allow show_notification to replace a given notification id

[William Friesen]

For some plugins (such as the Amarok plugin being discussed on the mailing list), it would be a good feature to be able to "update" notifications by replacing them with a new one.

[William Friesen (william-friesen)]

In the initial report above I incorrectly said the Amarok plugin, when I meant Volume Control. Here is a copy of the Volume Control plugin that demonstrates how this patch can be used. For example, when activating VolumeUp twice in succession, the first notification will be replaced, rather than adding another to the notification queue.

[X (u78qir8a9-deactivatedaccount)]

Thanks a lot, it's applied. I added a docstring to the function too