Continuously monitor synchronisation status

[tdltdc]

Currently, the current time is compared to the scheduled broadcast date and time of the next broadcast only once, when sbsc is launched.

It would be better to monitor this continuously so that when, for example, sbsc is launched 2 hours before the scheduled broadcast time of the next broadcast, the message would switch from a red "out of sync" message to a green "in sync" message after 75 minutes (i.e. when the time difference is less than 45 minutes).