fixes to gcs / flight plan editor

Fixes for issue #927 

- deleting a wp from Waypoint Edit dialog or from XML editor does not remove the item from the georefs menus
- changing the name of a wp in xml editor or in dialog does not update the menus
- closing a flight plan does not clear the menus
- opening a different plan after closing the first does not work because fit to window keeps trying to fix wp from closed fp


Other changes
- keep map center when zooming with the spin button
- When the georef is set to a wp and the mouse is moved to a different UTM zone it would no longer update the distance. Now it shows an error message. TODO: fix so that it works across utm zones.

NOT Fixed
- creating a new wp by inserting a new xml item or by copying an existing item does not add the wp to the window or the menus
- deleting a wp does not remove it from sectors
- changing the name of a wp does not update the sectors