multiple map folders

it would be nice to allow two or three different map folders where oruxmaps finds its offline maps. we could then have a "basic" supply of "small" maps (like the osm .map files) on the internal phone memory and carry the real stuff (like huge multigigabyte topographic maps) on an external sd card. this way, oruxmaps would still be useful, even when external_sd is unavailable for some reason.