Heuristics for when node or client is offline

[mitra42]

We want an indication of when a node, previously displayed, is offline.

• The idea is that we listen for something - this might be for a node, the discovery message, or for the client, just any message from the client.
• This then sets a lastseen field to the time
• And keeps a record of how often seen (in milliseconds)
• When it hasn't been seen for a time that exceeds some multiple of the typical frequency then its marked in grey
• The actual determination would be a function running on the Node and on the Client, so the could be tuned differently.

[mitra42]

Done but see #9