Changes mcb.LoadWorld to load from a file name. This is the
most common use case. It will also store the root path as
part of the World struct. Additionally this method creates
and stores a list of all region files associated with the world.

Neither the region file names nor the root path will persist to
the world.js file upon saving.

Fixes mcbconv to reflect the above change.
Adds a code to mcb to list region files.
Begins re-implementation of mcmapgen, after introduction of mcb file format.
We should no longer require any reference to the anvil or nbt packages at
this stage.