Map editor doesn't use -b sprites

[beccabunny]

All the planet sprites that were added in endless-sky/endless-sky#3458 and that finish with -b seem to never be used by the map editor when randomizing a system or a planet, which reduces differentiation quite a lot

[tehhowch]

The sprites and commodity lists (including stars) are currently hard coded in the editor. In the future these should be customizable, with defaults drawn from reading the base game locations.