Remove inventorycube() workaround for default:glass in minimal game

Catch some error situations when mod used without thinking about it

Most modders would otherwise forget to explicitly define this, and generated nodes aliased from mods would wall-off caves

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.