minetest.registered_ores lua table (and idem for decorations and biomes)

I've already made mapgens, and I wanted to put minerals in these mapgens. The problem is that we can't use the ores API to place ores. And there isn't any list of registered ores in LUA, I had to redefine all of the ores. So I've fixed it, and I've done the same for decorations and biomes.
So we have `minetest.registered_ores`, `minetest.registered_decorations` and `minetest.registered_biomes`.

In practice, the behaviour of the registration functions isn't changed.