Separate basic functions and game settings from "default" into a new mod

[CasimirKaPazi]

Not sure if this will be useful in the long therm. There has been a discussion in a Minetest Game issue.

The main purpose is to have the base and behavior the game provides in a dedicated mod and have "default" only for content. Functions can easily be aliased to default. Also move content out of default as separate mods. e.g. Trees, Furnace, Chests

layer 1	layer 2	layer 3
player		trees
sounds	default	furnace
functions		chests

Default then should only contain 1. subgame specific settings (like stack_max) 2. Nodes the mapgen provides (stone, dirt, water).