Release 4.1.6-rc3

We might have finally found the cause of the application hangs during shutdown.
Note that this became easier to track down due to the current (2021-01-11) downtime for migration of www.edsm.net. EDMarketConnector will hang during shutdown prior to this version whilst EDSM isn't available.

• Avoid making Tk event_generate() calls whilst the appliction is shutting down.

• Plugins should actively avoid making any sort of Tk event_generate() call during application shutdown.

  This means using if not config.shutting_down: to gate any code in worker threads that might attempt this. Also, be sure you're not attempting such in your plugin_stop() function.

  See plugins/edsm.py and plugins/inara.py for example of the usage.

• Any use of plug.show_error() won't actually change the UI status line during shutdown, but the text you tried to show will be logged instead.