commands.import_.srtm: add option to import local files
model.flight: speed up elevations query even more
flight.js: remove unneccessary .show() and .hide() when adding/removing a plane marker
flight.js: don't recreate the plane icons on every update
flight.js: clone point for plane icon

Otherwise moving the plane icon changes the linestring itself.
flight.js: use ol.LonLat instead of ol.Point for transformation

LonLat is more lightweight...