diffi: display date in red when the difference between the time of the last block and current time is too high

Note: It would be better to fix #216 (closed)

Display in red when the difference between the time of the last block and current time is too high >~ 2 hours.

This could help to know if the node or the network is stuck.

I don’t how to handle this, as the view is updated when a new blocks arrive.