Consider a synthetic `LastSeen` event for plugins

[Athanasius]

Currently after the latest Journal is loaded there is a synthetic StartUp event created, which plugins will receive, but only if the game was detected as running during catchup.

Assuming the latest Journal had an actual game login the main UI will show at least the Cmdr name, but unless the game was running this won't be passed to plugins. Plugins would need to wait for the game to be running (a necessary pre-condition for any new Journal events to be written, be it to the same file or a new one) to even know the Cmdr name.

So, perhaps in the "game was not running during catch up" case we should emit a new synthetic event, e.g. LastSeen (name up for debate), to pass that data.

Why not just emit StartUp anyway? Because the semantics of a plugin's journal_entry() being called is very much "this is new live data", and the current StartUp event matches that. If we're going to call the function with historic state/data, then it needs to be a new event so as to make that clear.