mintupdate: replace icons with better selection

The broken lock is currently used for errors but also when the state isn't known (for instance when there is no connection). This should be more discreet as it's not an error which needs to be acted upon immediately, but merely an indication of the fact that the system doesn't know whether it's up to date or not. So maybe a little question mark on top of the lock would be better?

Also, when the system checks, it could be better to show a little refresh mark on top of the lock. The meaning of the current closed lock isn't very obvious.