Add last status time to contact view

Up to gajim 0.16.9 it was possible to see the time since a contact changed the status to offline. I find it quite useful to see when a contact was "online".

So I wrote a patch (basically copy & paste the old code from gajim 0.16.9) for gajim 1.0.3 (and gajim master), which brings this feature back.