Remove stale notifications after an offline update has completed

https://bugzilla.gnome.org/show_bug.cgi?id=726645
Update the last installed timestamp after successful offline updates

This makes sure our internal timestamp is up to date after an external
app (e.g. gnome-shell) has triggered offline updates.

https://bugzilla.gnome.org/show_bug.cgi?id=726645
Fix showing recently installed updates

This fixes the state tracking code so that the updates page doesn't get
stuck at "Looking for new updates…" after clicking 'Review' on the
notification.

https://bugzilla.gnome.org/show_bug.cgi?id=726645
Improve display of recently installed updates

Don't show the number of updates in the tab button. This is
misleading since the button is not depressed, so having the
number there gives the impression that there is _another_ tab
with that number of updates, when in fact the number refers
to the already installed updates that we are showing.

Also, don't show a refresh button for already installed updates.