cubicweb #773449 update notification similar to tweeter's [open]

When you're on tweeter, displaying your "home" page, you see the latest short messages from the people you follow. Actually this list is somewhat "live" : periodically, you some message exchange is performed between the navigator and the server and in case new messages are available, a message is displayed in a clickable zone at the top of the page saying something like "N new tweets available", or if you are looking at a live search page "N new tweets since you started searching".

I'd like to have an easy way in CW to display such notices to the end users of an application.

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