src/mainfrm.cc: Update to call img_error2msg() on the error code
returned by img_error().
lib/survex.pot,src/: Hide the "View" box for GPX export.  Add
checkboxes for "Entrances", "Fixed Points" and "Exported Stations"
for GPX export (currently non-functional).  Prepare for allowing
text height, marker size, and grid spacing to be set from aven (as
cad3d allows).
src/: Rename internal references to "show splays" to just "splays"
to reflect the change in the menu item name.