Rename block ID -> index

The block _name_ is meant as the fixed "identifier", whereas the
"index" (formerly ID) is only intended to be used for rendering
or in-memory storage purposes.
Add getItemDisplayName(), get human-readable descriptive name

Useful for https://github.com/deathcap/voxel-voila/issues/1
and https://github.com/deathcap/inventory-window/issues/11
0.2.0