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.