plug-ins: let's special-case just a bit the HGT data.

- Display title "Digital Elevation Model data" instead of just "Image".
- Don't show the type combo anymore, since HGT is always 16-bit signed
  integer in big endian.
- Don't show the width and height either, but instead show a dropdown
  list to choose between either SRTM-1 or SRTM-3 data. There seems to be
  only these 2 types of data, and this will determine the width and
  height. This choice (SRTM-1 vs 3) will mostly likely be better known
  by people than knowing what image dimension they have to set,
  especially since official links for such files seem to all include the
  information.