Add notifications for events that deserve them.
If system update has happened, treat "update available" as
worthy of notification again.
Make "reboot required" messages work correctly, so that
after a successful update user can still check for updates,
and the old "reboot is required to finish the update"-message
stays (instead of telling user that the system is up-to-date).
This is done by actually checking if the default boot image
matches the one that is currently running.