Adding a LastSeen property to accesspoints and ScanDone signal

[Mathieu Trudel-Lapierre <mathieu trudel-lapierre canonical com>]

Hi,

We've been trying to use NetworkManager to get a better idea of the
reliability of wireless networks (their age, mostly) for use by
applications.

I expanded LastSeen to be exposed both over DBus for this purpose, as
well as in libnm-glib. I've also added a SCAN_DONE signal NMDeviceWifi,
again to be exposed via DBus. I did run into minor issues with this though,
and so patch 3/4 and 4/4 fix the supplicant sending extra ScanDone signals
to NMDeviceWifi, and how the last-seen property gets updated at the end
of a scan.

Regards,

--
Mathieu Trudel-Lapierre <mathieu trudel-lapierre canonical com>
Freenode: cyphermox, Jabber: mathieu tl gmail com
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