Implement a graphical stale image notification

[castrojo]

Right now if a user is on an image that's over 90 days old we put a warning in the terminal as a reminder to update. However this is only useful for people who use a terminal and totally not useful on handheld devices in game mode.

We should wire up user-motd.sh to also send something to libnotify so that a graphical desktop can get an out of date warning.

[Nitrousoxide]

Might be a good addition to add a notification if the ostree update processes fails since users intending to stay up to date could get out of date by 90 days if something goes wrong (like that key issue) and would be unaware until the stale image notification popped up.