multiple_flightpath_dockwidget triggers Exception

[joernu76]

After a TopView with a multiple_flightpath_dockwidget was closed, switching active flighttracks in the MainUI triggers an Exception.

This is caused by dangling connects referencing the deleted widget.

This may be resolved using the "viewCloses" event to "disconnect" every connection properly.

The whole signal-connections-event-lambda shenanigans should be reviewed.