update monitor: Split out a function

https://bugzilla.gnome.org/show_bug.cgi?id=728000
update monitor: Set install-timestamp in a single place

Instead of setting it both when triggering an offline update and after
the reboot, only do the latter. This makes sure we have one code path no
matter if it was gnome-software or gnome-shell triggering the update.

https://bugzilla.gnome.org/show_bug.cgi?id=728000
update monitor: Only show installed update notification once

We don't need to nag the user to review installed updates; if they
choose to dismiss the notification, respect that and avoid reshowing it
again at next login.

https://bugzilla.gnome.org/show_bug.cgi?id=728000
Revert "Disable the GTK pixel cache due to rendering artifacts"

This reverts commit b36f427055cec55707ac904024f53060db4de0a4 -- it's been fixed
in GTK now.