EDDB station link now triggers off this.system_population, which is
received via monitor passing 'Population' through in Startup, Location,
FSDJump and CarrierJump events.
One fewer use cases for systems.p (populated status in this case).
This shouldn't actually be necessary, the marketid shouln't change
because of a jump, and would have been picked up from the other events
anyway, but it should do no harm to set it again from this.
* Adds monitor.station_marketid, tracked at the same points as
monitor.station (name).
* Changes EDDB plugin to also track its own this.station_marketid so
that it can be used in /market-id/ EDDB URL.
This removes a use case for stations.p file.
Notify plugins when Cmdr or Beta status changes while the preferences dialog
is open.
Display a note on the Credentials tab when Cmdr is not set. Addresses #224
