Reacting to a feature request, attached patch adds support for
parkpos selection in the "location on ground" dialog, and clears any
parkpos preset in the "airports" dialog.
Make a new set location (on ground) dialog box which knows how to set up
the /sim/presets/ properties correctly for a new ground position.
For now leave the previous all-in-one dialog box for in-air resets. However
this really needs some more work to make sure the /sim/presets properties get
set correctly.