lib/survex.pot,src/export.cc,src/export.h,src/printwx.cc: Add
"Origin in centre" and "Full coordinates" options to export
- the latter fixes #10.  GPX and PLT output force "full
coordinates".
src/printwx.cc: Swap the "Elements" and "View" boxes in the print
and export dialogs, as that works much better when the "View" box
is hidden.